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733 lines
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733 lines
35 KiB
HTML
<!DOCTYPE HTML>
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<title>递归 - Go语言圣经</title>
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<nav id="sidebar" class="sidebar" aria-label="Table of contents">
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<div class="sidebar-scrollbox">
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<ol class="chapter">
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<li class="chapter-item expanded ">
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<a href="../index.html" >Go语言圣经</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../preface-zh.html" >译者序</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../preface.html" >前言</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1.html" ><strong aria-hidden="true">1.</strong> 入门</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-01.html" ><strong aria-hidden="true">1.1.</strong> Hello, World</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-02.html" ><strong aria-hidden="true">1.2.</strong> 命令行参数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-03.html" ><strong aria-hidden="true">1.3.</strong> 查找重复的行</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-04.html" ><strong aria-hidden="true">1.4.</strong> GIF动画</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-05.html" ><strong aria-hidden="true">1.5.</strong> 获取URL</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-06.html" ><strong aria-hidden="true">1.6.</strong> 并发获取多个URL</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-07.html" ><strong aria-hidden="true">1.7.</strong> Web服务</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-08.html" ><strong aria-hidden="true">1.8.</strong> 本章要点</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2.html" ><strong aria-hidden="true">2.</strong> 程序结构</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-01.html" ><strong aria-hidden="true">2.1.</strong> 命名</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-02.html" ><strong aria-hidden="true">2.2.</strong> 声明</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-03.html" ><strong aria-hidden="true">2.3.</strong> 变量</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-04.html" ><strong aria-hidden="true">2.4.</strong> 赋值</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-05.html" ><strong aria-hidden="true">2.5.</strong> 类型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-06.html" ><strong aria-hidden="true">2.6.</strong> 包和文件</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-07.html" ><strong aria-hidden="true">2.7.</strong> 作用域</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3.html" ><strong aria-hidden="true">3.</strong> 基础数据类型</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-01.html" ><strong aria-hidden="true">3.1.</strong> 整型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-02.html" ><strong aria-hidden="true">3.2.</strong> 浮点数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-03.html" ><strong aria-hidden="true">3.3.</strong> 复数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-04.html" ><strong aria-hidden="true">3.4.</strong> 布尔型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-05.html" ><strong aria-hidden="true">3.5.</strong> 字符串</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-06.html" ><strong aria-hidden="true">3.6.</strong> 常量</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4.html" ><strong aria-hidden="true">4.</strong> 复合数据类型</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-01.html" ><strong aria-hidden="true">4.1.</strong> 数组</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-02.html" ><strong aria-hidden="true">4.2.</strong> Slice</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-03.html" ><strong aria-hidden="true">4.3.</strong> Map</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-04.html" ><strong aria-hidden="true">4.4.</strong> 结构体</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-05.html" ><strong aria-hidden="true">4.5.</strong> JSON</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-06.html" ><strong aria-hidden="true">4.6.</strong> 文本和HTML模板</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5.html" ><strong aria-hidden="true">5.</strong> 函数</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-01.html" ><strong aria-hidden="true">5.1.</strong> 函数声明</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-02.html" class="active"><strong aria-hidden="true">5.2.</strong> 递归</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-03.html" ><strong aria-hidden="true">5.3.</strong> 多返回值</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-04.html" ><strong aria-hidden="true">5.4.</strong> 错误</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-05.html" ><strong aria-hidden="true">5.5.</strong> 函数值</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-06.html" ><strong aria-hidden="true">5.6.</strong> 匿名函数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-07.html" ><strong aria-hidden="true">5.7.</strong> 可变参数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-08.html" ><strong aria-hidden="true">5.8.</strong> Deferred函数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-09.html" ><strong aria-hidden="true">5.9.</strong> Panic异常</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-10.html" ><strong aria-hidden="true">5.10.</strong> Recover捕获异常</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6.html" ><strong aria-hidden="true">6.</strong> 方法</a>
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||
</li>
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||
<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-01.html" ><strong aria-hidden="true">6.1.</strong> 方法声明</a>
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||
</li>
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||
<li class="chapter-item expanded ">
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<a href="../ch6/ch6-02.html" ><strong aria-hidden="true">6.2.</strong> 基于指针对象的方法</a>
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</li>
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||
<li class="chapter-item expanded ">
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<a href="../ch6/ch6-03.html" ><strong aria-hidden="true">6.3.</strong> 通过嵌入结构体来扩展类型</a>
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</li>
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||
<li class="chapter-item expanded ">
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<a href="../ch6/ch6-04.html" ><strong aria-hidden="true">6.4.</strong> 方法值和方法表达式</a>
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||
</li>
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||
<li class="chapter-item expanded ">
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<a href="../ch6/ch6-05.html" ><strong aria-hidden="true">6.5.</strong> 示例: Bit数组</a>
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||
</li>
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||
<li class="chapter-item expanded ">
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<a href="../ch6/ch6-06.html" ><strong aria-hidden="true">6.6.</strong> 封装</a>
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||
</li>
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||
</ol>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7.html" ><strong aria-hidden="true">7.</strong> 接口</a>
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</li>
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||
<ol class="section">
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||
<li class="chapter-item expanded ">
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<a href="../ch7/ch7-01.html" ><strong aria-hidden="true">7.1.</strong> 接口是合约</a>
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||
</li>
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||
<li class="chapter-item expanded ">
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<a href="../ch7/ch7-02.html" ><strong aria-hidden="true">7.2.</strong> 接口类型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-03.html" ><strong aria-hidden="true">7.3.</strong> 实现接口的条件</a>
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||
</li>
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||
<li class="chapter-item expanded ">
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<a href="../ch7/ch7-04.html" ><strong aria-hidden="true">7.4.</strong> flag.Value接口</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-05.html" ><strong aria-hidden="true">7.5.</strong> 接口值</a>
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</li>
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||
<li class="chapter-item expanded ">
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<a href="../ch7/ch7-06.html" ><strong aria-hidden="true">7.6.</strong> sort.Interface接口</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-07.html" ><strong aria-hidden="true">7.7.</strong> http.Handler接口</a>
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</li>
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||
<li class="chapter-item expanded ">
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<a href="../ch7/ch7-08.html" ><strong aria-hidden="true">7.8.</strong> error接口</a>
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||
</li>
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||
<li class="chapter-item expanded ">
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<a href="../ch7/ch7-09.html" ><strong aria-hidden="true">7.9.</strong> 示例: 表达式求值</a>
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||
</li>
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||
<li class="chapter-item expanded ">
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<a href="../ch7/ch7-10.html" ><strong aria-hidden="true">7.10.</strong> 类型断言</a>
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||
</li>
|
||
<li class="chapter-item expanded ">
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||
<a href="../ch7/ch7-11.html" ><strong aria-hidden="true">7.11.</strong> 基于类型断言识别错误类型</a>
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||
</li>
|
||
<li class="chapter-item expanded ">
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<a href="../ch7/ch7-12.html" ><strong aria-hidden="true">7.12.</strong> 通过类型断言查询接口</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
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||
<a href="../ch7/ch7-13.html" ><strong aria-hidden="true">7.13.</strong> 类型分支</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
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||
<a href="../ch7/ch7-14.html" ><strong aria-hidden="true">7.14.</strong> 示例: 基于标记的XML解码</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
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<a href="../ch7/ch7-15.html" ><strong aria-hidden="true">7.15.</strong> 补充几点</a>
|
||
</li>
|
||
</ol>
|
||
<li class="chapter-item expanded ">
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||
<a href="../ch8/ch8.html" ><strong aria-hidden="true">8.</strong> Goroutines和Channels</a>
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||
</li>
|
||
<ol class="section">
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch8/ch8-01.html" ><strong aria-hidden="true">8.1.</strong> Goroutines</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch8/ch8-02.html" ><strong aria-hidden="true">8.2.</strong> 示例: 并发的Clock服务</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch8/ch8-03.html" ><strong aria-hidden="true">8.3.</strong> 示例: 并发的Echo服务</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch8/ch8-04.html" ><strong aria-hidden="true">8.4.</strong> Channels</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch8/ch8-05.html" ><strong aria-hidden="true">8.5.</strong> 并发的循环</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch8/ch8-06.html" ><strong aria-hidden="true">8.6.</strong> 示例: 并发的Web爬虫</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch8/ch8-07.html" ><strong aria-hidden="true">8.7.</strong> 基于select的多路复用</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch8/ch8-08.html" ><strong aria-hidden="true">8.8.</strong> 示例: 并发的目录遍历</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch8/ch8-09.html" ><strong aria-hidden="true">8.9.</strong> 并发的退出</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch8/ch8-10.html" ><strong aria-hidden="true">8.10.</strong> 示例: 聊天服务</a>
|
||
</li>
|
||
</ol>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch9/ch9.html" ><strong aria-hidden="true">9.</strong> 基于共享变量的并发</a>
|
||
</li>
|
||
<ol class="section">
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch9/ch9-01.html" ><strong aria-hidden="true">9.1.</strong> 竞争条件</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch9/ch9-02.html" ><strong aria-hidden="true">9.2.</strong> sync.Mutex互斥锁</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch9/ch9-03.html" ><strong aria-hidden="true">9.3.</strong> sync.RWMutex读写锁</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch9/ch9-04.html" ><strong aria-hidden="true">9.4.</strong> 内存同步</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch9/ch9-05.html" ><strong aria-hidden="true">9.5.</strong> sync.Once惰性初始化</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch9/ch9-06.html" ><strong aria-hidden="true">9.6.</strong> 竞争条件检测</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch9/ch9-07.html" ><strong aria-hidden="true">9.7.</strong> 示例: 并发的非阻塞缓存</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch9/ch9-08.html" ><strong aria-hidden="true">9.8.</strong> Goroutines和线程</a>
|
||
</li>
|
||
</ol>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10.html" ><strong aria-hidden="true">10.</strong> 包和工具</a>
|
||
</li>
|
||
<ol class="section">
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-01.html" ><strong aria-hidden="true">10.1.</strong> 包简介</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-02.html" ><strong aria-hidden="true">10.2.</strong> 导入路径</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-03.html" ><strong aria-hidden="true">10.3.</strong> 包声明</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-04.html" ><strong aria-hidden="true">10.4.</strong> 导入声明</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-05.html" ><strong aria-hidden="true">10.5.</strong> 包的匿名导入</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-06.html" ><strong aria-hidden="true">10.6.</strong> 包和命名</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-07.html" ><strong aria-hidden="true">10.7.</strong> 工具</a>
|
||
</li>
|
||
</ol>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch11/ch11.html" ><strong aria-hidden="true">11.</strong> 测试</a>
|
||
</li>
|
||
<ol class="section">
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch11/ch11-01.html" ><strong aria-hidden="true">11.1.</strong> go test</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch11/ch11-02.html" ><strong aria-hidden="true">11.2.</strong> 测试函数</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch11/ch11-03.html" ><strong aria-hidden="true">11.3.</strong> 测试覆盖率</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch11/ch11-04.html" ><strong aria-hidden="true">11.4.</strong> 基准测试</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch11/ch11-05.html" ><strong aria-hidden="true">11.5.</strong> 剖析</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch11/ch11-06.html" ><strong aria-hidden="true">11.6.</strong> 示例函数</a>
|
||
</li>
|
||
</ol>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12.html" ><strong aria-hidden="true">12.</strong> 反射</a>
|
||
</li>
|
||
<ol class="section">
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-01.html" ><strong aria-hidden="true">12.1.</strong> 为何需要反射?</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-02.html" ><strong aria-hidden="true">12.2.</strong> reflect.Type和reflect.Value</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-03.html" ><strong aria-hidden="true">12.3.</strong> Display递归打印</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-04.html" ><strong aria-hidden="true">12.4.</strong> 示例: 编码S表达式</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-05.html" ><strong aria-hidden="true">12.5.</strong> 通过reflect.Value修改值</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-06.html" ><strong aria-hidden="true">12.6.</strong> 示例: 解码S表达式</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-07.html" ><strong aria-hidden="true">12.7.</strong> 获取结构体字段标签</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-08.html" ><strong aria-hidden="true">12.8.</strong> 显示一个类型的方法集</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-09.html" ><strong aria-hidden="true">12.9.</strong> 几点忠告</a>
|
||
</li>
|
||
</ol>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch13/ch13.html" ><strong aria-hidden="true">13.</strong> 底层编程</a>
|
||
</li>
|
||
<ol class="section">
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch13/ch13-01.html" ><strong aria-hidden="true">13.1.</strong> unsafe.Sizeof, Alignof 和 Offsetof</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch13/ch13-02.html" ><strong aria-hidden="true">13.2.</strong> unsafe.Pointer</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch13/ch13-03.html" ><strong aria-hidden="true">13.3.</strong> 示例: 深度相等判断</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch13/ch13-04.html" ><strong aria-hidden="true">13.4.</strong> 通过cgo调用C代码</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch13/ch13-05.html" ><strong aria-hidden="true">13.5.</strong> 几点忠告</a>
|
||
</li>
|
||
</ol>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../appendix/appendix.html" ><strong aria-hidden="true">14.</strong> 附录</a>
|
||
</li>
|
||
<ol class="section">
|
||
<li class="chapter-item expanded ">
|
||
<a href="../appendix/appendix-a-errata.html" ><strong aria-hidden="true">14.1.</strong> 附录A：原文勘误</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../appendix/appendix-b-author.html" ><strong aria-hidden="true">14.2.</strong> 附录B：作者译者</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../appendix/appendix-c-cpoyright.html" ><strong aria-hidden="true">14.3.</strong> 附录C：译文授权</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../appendix/appendix-d-translations.html" ><strong aria-hidden="true">14.4.</strong> 附录D：其它语言</a>
|
||
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|
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<ul dir="auto"><li><em>凹语言(Go实现, 面向WASM设计): <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li><li><em>MnBook(Go语言实现的MD电子书构建工具): <a href="https://github.com/wa-lang/mnbook">https://github.com/wa-lang/mnbook</a></em></li></ul><hr>
|
||
|
||
<h2>5.2. 递归</h2>
|
||
<p>函数可以是递归的，这意味着函数可以直接或间接的调用自身。对许多问题而言，递归是一种强有力的技术，例如处理递归的数据结构。在4.4节，我们通过遍历二叉树来实现简单的插入排序，在本章节，我们再次使用它来处理HTML文件。</p>
|
||
<p>下文的示例代码使用了非标准包 golang.org/x/net/html ，解析HTML。golang.org/x/... 目录下存储了一些由Go团队设计、维护，对网络编程、国际化文件处理、移动平台、图像处理、加密解密、开发者工具提供支持的扩展包。未将这些扩展包加入到标准库原因有二，一是部分包仍在开发中，二是对大多数Go语言的开发者而言，扩展包提供的功能很少被使用。</p>
|
||
<p>例子中调用golang.org/x/net/html的部分api如下所示。html.Parse函数读入一组bytes解析后，返回html.Node类型的HTML页面树状结构根节点。HTML拥有很多类型的结点如text（文本）、comments（注释）类型，在下面的例子中，我们 只关注< name key='value' >形式的结点。</p>
|
||
<p><!-- raw HTML omitted --><!-- raw HTML omitted -->golang.org/x/net/html<!-- raw HTML omitted --><!-- raw HTML omitted --></p>
|
||
<pre><code class="language-Go">package html
|
||
|
||
type Node struct {
|
||
Type NodeType
|
||
Data string
|
||
Attr []Attribute
|
||
FirstChild, NextSibling *Node
|
||
}
|
||
|
||
type NodeType int32
|
||
|
||
const (
|
||
ErrorNode NodeType = iota
|
||
TextNode
|
||
DocumentNode
|
||
ElementNode
|
||
CommentNode
|
||
DoctypeNode
|
||
)
|
||
|
||
type Attribute struct {
|
||
Key, Val string
|
||
}
|
||
|
||
func Parse(r io.Reader) (*Node, error)
|
||
</code></pre>
|
||
<p>main函数解析HTML标准输入，通过递归函数visit获得links（链接），并打印出这些links：</p>
|
||
<p><!-- raw HTML omitted --><!-- raw HTML omitted -->gopl.io/ch5/findlinks1<!-- raw HTML omitted --><!-- raw HTML omitted --></p>
|
||
<pre><code class="language-Go">// Findlinks1 prints the links in an HTML document read from standard input.
|
||
package main
|
||
|
||
import (
|
||
"fmt"
|
||
"os"
|
||
|
||
"golang.org/x/net/html"
|
||
)
|
||
|
||
func main() {
|
||
doc, err := html.Parse(os.Stdin)
|
||
if err != nil {
|
||
fmt.Fprintf(os.Stderr, "findlinks1: %v\n", err)
|
||
os.Exit(1)
|
||
}
|
||
for _, link := range visit(nil, doc) {
|
||
fmt.Println(link)
|
||
}
|
||
}
|
||
</code></pre>
|
||
<p>visit函数遍历HTML的节点树，从每一个anchor元素的href属性获得link,将这些links存入字符串数组中，并返回这个字符串数组。</p>
|
||
<pre><code class="language-Go">// visit appends to links each link found in n and returns the result.
|
||
func visit(links []string, n *html.Node) []string {
|
||
if n.Type == html.ElementNode && n.Data == "a" {
|
||
for _, a := range n.Attr {
|
||
if a.Key == "href" {
|
||
links = append(links, a.Val)
|
||
}
|
||
}
|
||
}
|
||
for c := n.FirstChild; c != nil; c = c.NextSibling {
|
||
links = visit(links, c)
|
||
}
|
||
return links
|
||
}
|
||
</code></pre>
|
||
<p>为了遍历结点n的所有后代结点，每次遇到n的孩子结点时，visit递归的调用自身。这些孩子结点存放在FirstChild链表中。</p>
|
||
<p>让我们以Go的主页（golang.org）作为目标，运行findlinks。我们以fetch（1.5章）的输出作为findlinks的输入。下面的输出做了简化处理。</p>
|
||
<pre><code>$ go build gopl.io/ch1/fetch
|
||
$ go build gopl.io/ch5/findlinks1
|
||
$ ./fetch https://golang.org | ./findlinks1
|
||
#
|
||
/doc/
|
||
/pkg/
|
||
/help/
|
||
/blog/
|
||
http://play.golang.org/
|
||
//tour.golang.org/
|
||
https://golang.org/dl/
|
||
//blog.golang.org/
|
||
/LICENSE
|
||
/doc/tos.html
|
||
http://www.google.com/intl/en/policies/privacy/
|
||
</code></pre>
|
||
<p>注意在页面中出现的链接格式，在之后我们会介绍如何将这些链接，根据根路径（ <a href="https://golang.org">https://golang.org</a> ）生成可以直接访问的url。</p>
|
||
<p>在函数outline中，我们通过递归的方式遍历整个HTML结点树，并输出树的结构。在outline内部，每遇到一个HTML元素标签，就将其入栈，并输出。</p>
|
||
<p><!-- raw HTML omitted --><!-- raw HTML omitted -->gopl.io/ch5/outline<!-- raw HTML omitted --><!-- raw HTML omitted --></p>
|
||
<pre><code class="language-Go">func main() {
|
||
doc, err := html.Parse(os.Stdin)
|
||
if err != nil {
|
||
fmt.Fprintf(os.Stderr, "outline: %v\n", err)
|
||
os.Exit(1)
|
||
}
|
||
outline(nil, doc)
|
||
}
|
||
func outline(stack []string, n *html.Node) {
|
||
if n.Type == html.ElementNode {
|
||
stack = append(stack, n.Data) // push tag
|
||
fmt.Println(stack)
|
||
}
|
||
for c := n.FirstChild; c != nil; c = c.NextSibling {
|
||
outline(stack, c)
|
||
}
|
||
}
|
||
</code></pre>
|
||
<p>有一点值得注意：outline有入栈操作，但没有相对应的出栈操作。当outline调用自身时，被调用者接收的是stack的拷贝。被调用者对stack的元素追加操作，修改的是stack的拷贝，其可能会修改slice底层的数组甚至是申请一块新的内存空间进行扩容；但这个过程并不会修改调用方的stack。因此当函数返回时，调用方的stack与其调用自身之前完全一致。</p>
|
||
<p>下面是 <a href="https://golang.org">https://golang.org</a> 页面的简要结构:</p>
|
||
<pre><code>$ go build gopl.io/ch5/outline
|
||
$ ./fetch https://golang.org | ./outline
|
||
[html]
|
||
[html head]
|
||
[html head meta]
|
||
[html head title]
|
||
[html head link]
|
||
[html body]
|
||
[html body div]
|
||
[html body div]
|
||
[html body div div]
|
||
[html body div div form]
|
||
[html body div div form div]
|
||
[html body div div form div a]
|
||
...
|
||
</code></pre>
|
||
<p>正如你在上面实验中所见，大部分HTML页面只需几层递归就能被处理，但仍然有些页面需要深层次的递归。</p>
|
||
<p>大部分编程语言使用固定大小的函数调用栈，常见的大小从64KB到2MB不等。固定大小栈会限制递归的深度，当你用递归处理大量数据时，需要避免栈溢出；除此之外，还会导致安全性问题。与此相反，Go语言使用可变栈，栈的大小按需增加（初始时很小）。这使得我们使用递归时不必考虑溢出和安全问题。</p>
|
||
<p><strong>练习 5.1：</strong> 修改findlinks代码中遍历n.FirstChild链表的部分，将循环调用visit，改成递归调用。</p>
|
||
<p><strong>练习 5.2：</strong> 编写函数，记录在HTML树中出现的同名元素的次数。</p>
|
||
<p><strong>练习 5.3：</strong> 编写函数输出所有text结点的内容。注意不要访问<code><script></code>和<code><style></code>元素，因为这些元素对浏览者是不可见的。</p>
|
||
<p><strong>练习 5.4：</strong> 扩展visit函数，使其能够处理其他类型的结点，如images、scripts和style sheets。</p>
|
||
|
||
|
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