LeetCode 32. Longest Valid Parentheses

Given a string containing just the characters '(' and ')', find the length of the longest valid (well-formed) parentheses substring.

Example 1:

Input: "(()"
Output: 2
Explanation: The longest valid parentheses substring is "()"

Example 2:

Input: ")()())"
Output: 4
Explanation: The longest valid parentheses substring is "()()"

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Seen this question in a real interview before?  

 

这道题比之“验证括号是否有效”要难一些，但是思路类似，也是借助栈，栈中村的是括号的下标，遇到左括号将其下标入栈；遇到右括号将栈定下标弹出并计算有效长度，同时记录下来当前最大长度，这里为了方便起见，我们一开始在栈中放了下标-1，用来表示最开始的位置，时间和空间复杂度都是O(N)

类似的题目有Valid Parentheses，Generate Parentheses

class Solution {
public:
    int longestValidParentheses(string s) {
        stack<int> sta;
        int maxRes = 0;
        sta.push(-1);
        for (int i = 0; i < s.size(); i++ )
        {
            if (s[i] == '(')
                sta.push(i);
            else
            {
                sta.pop();
                if (sta.empty())
                    sta.push(i);
                else
                    maxRes = max(maxRes, i - sta.top());
            }
        }
        return maxRes;
    }
};

这道题更详细的解答可以参考官方的solution，还有一种可以在O(1)空间复杂度完成的解法，有些难想，详情可以参考https://leetcode.com/problems/longest-valid-parentheses/solution/#