update at 2021-12-25 21:55:15 by ehlxr

budd-common/src/main/java/io/github/ehlxr/datastructure/queue/CircularQueue.java

@@ -0,0 +1,101 @@
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright © 2020 xrv <xrg@live.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package io.github.ehlxr.datastructure.queue;
+
+/**
+ * 数组实现循环队列
+ * <p>
+ * 一般情况下只需要 tail + 1 == head 即可判定队列已满，结合特殊情况（head == 0，tail == n - 1）
+ * 可使用公式 (tail + 1) % size == head 判定队列已满，head == tail 判断队列为空，
+ * 但这种公式判断循环队列会浪费一个数组的存储空间。
+ * <p>
+ * 另也可使用一个变量 n 记录当前队列中的数量（入队 +1，出队 -1），当 head == tail 时，
+ * 如果 n == size，则队列已满，如果 n == 0 则队列为空。
+ *
+ * @author ehlxr
+ * @since 2021-12-25 21:01.
+ */
+public class CircularQueue {
+    private final Integer[] data;
+    private int head = 0;
+    private int tail = 0;
+    private final int size;
+
+    public CircularQueue(int size) {
+        data = new Integer[size];
+        this.size = size;
+    }
+
+    public static void main(String[] args) {
+        CircularQueue queue = new CircularQueue(5);
+
+        // 循环队列会浪费一个数组的存储空间。队列容量为 5，入队 5 个最后一个失败
+        for (int i = 0; i < 5; i++) {
+            System.out.println(queue.enqueue(i));
+        }
+
+        // 出队 2 个，然后还可以入队 2 个
+        for (int i = 0; i < 2; i++) {
+            System.out.println(queue.dequeue());
+        }
+        System.out.println(queue.enqueue(7));
+        System.out.println(queue.enqueue(8));
+        System.out.println(queue.enqueue(9));
+
+        // 出队 6 个，最后一个失败
+        for (int i = 0; i < 6; i++) {
+            System.out.println(queue.dequeue());
+        }
+    }
+
+    /**
+     * 入队（从队尾添加元素）
+     */
+    public boolean enqueue(Integer item) {
+        // 判断队列是否已满
+        if ((tail + 1) % size == head) {
+            return false;
+        }
+        data[tail] = item;
+        tail = (tail + 1) % size;
+
+        return true;
+    }
+
+    /**
+     * 出队（从队首取出元素）
+     */
+    public Integer dequeue() {
+        // 判断队列是否为空
+        if (head == tail) {
+            return null;
+        }
+
+        Integer no = data[head];
+        head = (head + 1) % size;
+
+        return no;
+    }
+}