> 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/stack.md).

# 栈

### 栈的特点

**后进先出（LIFO）**：最后入栈的元素最先出栈。只能在栈顶操作。

### 数组实现

下标从 0 开始，`top` 指向栈顶元素，空栈时 `top = -1`：

```cpp
const int MAXN = 100;
int a[MAXN];
int top = -1;                 // 空栈

bool empty() { return top == -1; }
bool full()  { return top == MAXN - 1; }

void push(int x) { if (!full()) a[++top] = x; }   // 入栈
void pop()       { if (!empty()) top--; }         // 出栈
int  getTop()    { return a[top]; }               // 栈顶
```

> 原笔记用 `if (top < 芜湖)` 是不可运行的占位写法，容量要用常量。

### STL stack（推荐）

```cpp
#include <stack>
stack<int> st;
st.push(1);        // 入栈
st.top();          // 栈顶
st.pop();          // 出栈（不返回值）
st.size();         // 元素个数
st.empty();        // 是否为空
```

### 应用

* 括号匹配
* 表达式求值 / 中缀转后缀
* 函数的递归调用（系统栈）
* 单调栈（求下一个更大元素）

### 括号匹配示例

```cpp
bool check(string s) {
    stack<char> st;
    for (char c : s) {
        if (c == '(') st.push(c);
        else if (c == ')') {
            if (st.empty()) return false;
            st.pop();
        }
    }
    return st.empty();
}
```


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