# 18. 最长有效括号

java

```java
package leetcode.all.solution1_100;

import java.util.Stack;

/**
 * 32. 最长有效括号
 *
 * 给定一个只包含 '(' 和 ')' 的字符串，找出最长的包含有效括号的子串的长度。
 *
 * 示例 1:
 *
 * 输入: "(()"
 * 输出: 2
 * 解释: 最长有效括号子串为 "()"
 * 示例 2:
 *
 * 输入: ")()())"
 * 输出: 4
 * 解释: 最长有效括号子串为 "()()"
 */
public class Solution32 {
    /**
     * c表示当前未被匹配的左括号的数量，-1表示左括号，1表示右括号
     * 如果是左括号，将-1压入栈中
     * 如果是右括号：
     *      如果c==0,表示此时没有未被匹配的左括号，直接将1压入栈中即可
     *      c!=0，找到距离栈顶最近的元素，如果栈顶是2，4..6这样的数字应该将累计，同时c--
     *
     * @param s
     * @return
     */
    public int longestValidParentheses(String s) {
        int max=0;
        Stack<Integer> stack=new Stack<>();
        int c=0;//当前未被匹配的左括号的数量
        for(int i=0;i<s.length();i++){
            char ch=s.charAt(i);
            if(ch=='('){
                stack.push(-1);
                c++;
            }else{
                if(c==0) stack.push(1);
                else{
                    c--;
                    int flag=0;
                    while (stack.peek()!=-1){
                        flag+=stack.peek();
                        stack.pop();
                    }
                    stack.pop();
                    stack.push(flag+2);
                }
            }
        }

        int tmp=0;
        while(!stack.isEmpty()){
            int x=stack.pop();
            if(x!=-1&&x!=1){
                tmp+=x;
            }else{
                max=Math.max(tmp,max);
                tmp=0;
            }
        }
        max=Math.max(max,tmp);
        return max;
    }
    public static void main(String[] args){
        String s="()(())(";
        int r=new Solution32().longestValidParentheses(s);

        System.out.println();
        System.out.println("非动态规划:"+r);
        //b[]:0 2 0 0 2 6 0
    }
}
```

python

```
```


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