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179 lines
4.0 KiB
Markdown
179 lines
4.0 KiB
Markdown
---
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comments: true
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---
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# 线性查找
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「线性查找 Linear Search」是一种最基础的查找方法,其从数据结构的一端开始,依次访问每个元素,直到另一端后停止。
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## 算法实现
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线性查找实质上就是遍历数据结构 + 判断条件。比如,我们想要在数组 `nums` 中查找目标元素 `target` 的对应索引,那么可以在数组中进行线性查找。
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![linear_search](linear_search.assets/linear_search.png)
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=== "Java"
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```java title="linear_search.java"
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/* 线性查找(数组) */
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int linearSearch(int[] nums, int target) {
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// 遍历数组
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for (int i = 0; i < nums.length; i++) {
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// 找到目标元素,返回其索引
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if (nums[i] == target)
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return i;
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}
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// 未找到目标元素,返回 -1
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return -1;
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}
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```
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=== "C++"
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```cpp title="linear_search.cpp"
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/* 线性查找(数组) */
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int linearSearch(vector<int>& nums, int target) {
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// 遍历数组
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for (int i = 0; i < nums.size(); i++) {
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// 找到目标元素,返回其索引
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if (nums[i] == target)
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return i;
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}
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// 未找到目标元素,返回 -1
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return -1;
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}
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```
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=== "Python"
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```python title="linear_search.py"
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""" 线性查找(数组) """
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def linear_search(nums, target):
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# 遍历数组
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for i in range(len(nums)):
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if nums[i] == target: # 找到目标元素,返回其索引
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return i
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return -1 # 未找到目标元素,返回 -1
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```
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=== "Go"
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```go title="linear_search.go"
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```
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=== "JavaScript"
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```js title="linear_search.js"
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```
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=== "TypeScript"
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```typescript title="linear_search.ts"
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```
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=== "C"
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```c title="linear_search.c"
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```
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=== "C#"
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```csharp title="linear_search.cs"
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```
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再比如,我们想要在给定一个目标结点值 `target` ,返回此结点对象,也可以在链表中进行线性查找。
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=== "Java"
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```java title="linear_search.java"
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/* 线性查找(链表) */
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ListNode linearSearch(ListNode head, int target) {
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// 遍历链表
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while (head != null) {
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// 找到目标结点,返回之
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if (head.val == target)
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return head;
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head = head.next;
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}
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// 未找到目标结点,返回 null
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return null;
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}
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```
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=== "C++"
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```cpp title="linear_search.cpp"
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/* 线性查找(链表) */
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ListNode* linearSearch(ListNode* head, int target) {
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// 遍历链表
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while (head != nullptr) {
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// 找到目标结点,返回之
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if (head->val == target)
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return head;
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head = head->next;
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}
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// 未找到目标结点,返回 nullptr
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return nullptr;
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}
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```
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=== "Python"
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```python title="linear_search.py"
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""" 线性查找(链表) """
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def linear_search1(head, target):
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# 遍历链表
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while head:
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if head.val == target: # 找到目标结点,返回之
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return head
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head = head.next
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return None # 未找到目标结点,返回 None
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```
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=== "Go"
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```go title="linear_search.go"
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```
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=== "JavaScript"
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```js title="linear_search.js"
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```
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=== "TypeScript"
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```typescript title="linear_search.ts"
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```
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=== "C"
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```c title="linear_search.c"
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```
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=== "C#"
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```csharp title="linear_search.cs"
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```
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## 复杂度分析
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**时间复杂度 $O(n)$ :** 其中 $n$ 为数组或链表长度。
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**空间复杂度 $O(1)$ :** 无需使用额外空间。
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## 优缺点
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**线性查找的通用性极佳。** 由于线性查找是依次访问元素的,即没有跳跃访问元素,因此数组或链表皆适用。
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**线性查找的时间复杂度太高。** 在数据量 $n$ 很大时,查找效率很低。
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