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，可是思路相似，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，咱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)

相似的题目有Valid Parentheses，Generate Parentheses

 1 class Solution {
 2 public:
 3     int longestValidParentheses(string s) {
 4         stack<int> sta;
 5         int maxRes = 0;
 6         sta.push(-1);
 7         for (int i = 0; i < s.size(); i++ )
 8         {
 9             if (s[i] == '(')
10                 sta.push(i);
11             else
12             {
13                 sta.pop();
14                 if (sta.empty())
15                     sta.push(i);
16                 else
17                     maxRes = max(maxRes, i - sta.top());
18             }
19         }
20         return maxRes;
21     }
22 };

这道题更详细的解答能够参考官方的solution，还有一种能够在O(1)空间复杂度完成的解法，有些难想，详情能够参考https://leetcode.com/problems/longest-valid-parentheses/solution/#