> For the complete documentation index, see [llms.txt](https://docs.1l.lol/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.1l.lol/algorithm-techniques/two-pointers.md).

# 双指针与滑动窗口

### 相向双指针

两个指针从两端向中间移动，常用于有序数组求和、反转、回文判断。

```cpp
// 有序数组中找两数之和等于 target
int l = 0, r = n - 1;
while (l < r) {
    int s = a[l] + a[r];
    if (s == target) break;
    else if (s < target) l++;
    else r--;
}
```

### 同向双指针 / 滑动窗口

维护区间 \[l, r]，右指针扩张、左指针收缩，解决“连续子数组”问题。

```cpp
// 和 >= target 的最短连续子数组长度
int l = 0, sum = 0, ans = INF;
for (int r = 0; r < n; r++) {
    sum += a[r];
    while (sum >= target) {
        ans = min(ans, r - l + 1);
        sum -= a[l++];
    }
}
```

### 适用条件

数组有序或窗口具有**单调性**，一般把 O(n²) 降到 O(n)。

### 应用

两数/三数之和、最短/最长子数组、字符串匹配、去重。


---

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