# 最长有效括号

<p>给你一个只包含 <code>'('</code> 和 <code>')'</code> 的字符串，找出最长有效（格式正确且连续）括号子串的长度。</p><p> </p><div class="original__bRMd"><div><p><strong>示例 1：</strong></p><pre><strong>输入：</strong>s = "(()"<strong><br />输出：</strong>2<strong><br />解释：</strong>最长有效括号子串是 "()"</pre><p><strong>示例 2：</strong></p><pre><strong>输入：</strong>s = ")()())"<strong><br />输出：</strong>4<strong><br />解释：</strong>最长有效括号子串是 "()()"</pre><p><strong>示例 3：</strong></p><pre><strong>输入：</strong>s = ""<strong><br />输出：</strong>0</pre><p> </p><p><strong>提示：</strong></p><ul>	<li><code>0 <= s.length <= 3 * 10<sup>4</sup></code></li>	<li><code>s[i]</code> 为 <code>'('</code> 或 <code>')'</code></li></ul></div></div>

以下程序实现了这一功能，请你填补空白处的内容：

```cpp
#include <bits/stdc++.h>
using namespace std;
class Solution
{
public:
	int longestValidParentheses(string s)
	{
		stack<int> stk;
		int invalid = -1;
		int len = 0, max_len = 0;
		for (int i = 0; i < s.length(); i++)
		{
			if (s[i] == '(')
			{
				stk.push(i);
			}
			else
			{
				if (stk.empty())
				{
					invalid = i;
				}
				else
				{
					stk.pop();
					___________________;
				}
			}
		}
		return max_len;
	}
};
```

## template

```cpp
#include <bits/stdc++.h>
using namespace std;
class Solution
{
public:
	int longestValidParentheses(string s)
	{
		stack<int> stk;
		int invalid = -1;
		int len = 0, max_len = 0;
		for (int i = 0; i < s.length(); i++)
		{
			if (s[i] == '(')
			{
				stk.push(i);
			}
			else
			{
				if (stk.empty())
				{
					invalid = i;
				}
				else
				{
					stk.pop();
					if (stk.empty())
					{
						max_len = max(i - invalid, max_len);
					}
					else
					{
						max_len = max(i - stk.top(), max_len);
					}
				}
			}
		}
		return max_len;
	}
};
```

## 答案

```cpp
if (stk.empty())
{
	max_len = max(i - invalid, max_len);
}
else
{
	max_len = max(i - stk.top(), max_len);
}
```

## 选项

### A

```cpp
if (stk.empty())
{
	max_len = max(i - invalid, max_len);
}
else
{
	max_len = max(stk.top(), max_len);
}
```

### B

```cpp
if (stk.empty())
{
	max_len = max(invalid, max_len);
}
else
{
	max_len = max(i - stk.top(), max_len);
}
```

### C

```cpp
if (stk.empty())
{
	max_len = max(invalid - i, max_len);
}
else
{
	max_len = max(i - stk.top(), max_len);
}
```