# 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，请你填补空白处内容：

```java
import java.util.*;

class Solution {
	public int longestValidParentheses(String s) {
		int left = 0, right = 0, max = 0;
		for (int i = 0; i < s.length(); i++) {
			if (s.charAt(i) == '(')
				left++;
			else
				right++;
			if (left == right)
				max = Math.max(max, left * 2);
			if (right > left)
				left = right = 0;
		}
		left = 0;
		right = 0;
		for (int i = s.length() - 1; i >= 0; i--) {
			__________________;
			if (left == right)
				max = Math.max(max, left * 2);
			if (right < left)
				left = right = 0;
		}
		return max;
	}
}

```

## template

```java
import java.util.*;

class Solution {
	public int longestValidParentheses(String s) {
		int left = 0, right = 0, max = 0;
		for (int i = 0; i < s.length(); i++) {
			if (s.charAt(i) == '(')
				left++;
			else
				right++;
			if (left == right)
				max = Math.max(max, left * 2);
			if (right > left)
				left = right = 0;
		}
		left = 0;
		right = 0;
		for (int i = s.length() - 1; i >= 0; i--) {
			if (s.charAt(i) == '(')
				left++;
			else
				right++;
			if (left == right)
				max = Math.max(max, left * 2);
			if (right < left)
				left = right = 0;
		}
		return max;
	}
}

```

## 答案

```java
if (s.charAt(i) == '(')
	left++;
else
	right++;
```

## 选项

### A

```java
if (s.charAt(i) == ')')
	left++;
else
	right++;
```

### B

```java
if (s.charAt(i) == '(')
	right++;
else
	left++;
```

### C

```java
if (s.charAt(i) == ')')
	right++;
else
	left++;
```