> 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/data-structures/queue.md).

# 队列

### 队列的特点

**先进先出（FIFO）**：最先入队的元素最先出队。队尾入队，队头出队。

### 数组实现

```cpp
const int MAXN = 100;
int q[MAXN];
int front = 0, rear = 0;      // [front, rear) 区间存放元素

bool empty() { return front == rear; }
int  size()  { return rear - front; }

void push(int x) { q[rear++] = x; }     // 入队
void pop()       { if (!empty()) front++; }  // 出队
int  getFront()  { return q[front]; }   // 队头
```

> 原笔记的 `int q[5]` 长度写死、且被截断，容量应用常量。

### STL queue（推荐）

```cpp
#include <queue>
queue<int> q;
q.push(1);        // 入队
q.front();        // 队头
q.back();         // 队尾
q.pop();          // 出队（不返回值）
q.size();
q.empty();
```

### 普通队列的问题

出队只移动 `front`，前面的空间被浪费。反复使用时应改用**循环队列**（见《循环队列》）。

### 应用

* BFS 广度优先搜索
* 模拟排队、打印任务
* 滑动窗口 / 单调队列

### BFS 里的队列

```cpp
queue<int> q;
q.push(start);
vis[start] = true;
while (!q.empty()) {
    int u = q.front(); q.pop();
    for (int v : adj[u])
        if (!vis[v]) { vis[v] = true; q.push(v); }
}
```


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