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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這道題比之「驗證括號是否有效」要難一些，可是思路相似，也是藉助棧，棧中村的是括號的下標，遇到左括號將其下標入棧；遇到右括號將棧定下標彈出並計算有效長度，同時記錄下來當前最大長度，這裏爲了方便起見，咱們一開始在棧中放了下標-1，用來表示最開始的位置，時間和空間複雜度都是O(N)

相似的題目有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 };

這道題更詳細的解答能夠參考官方的solution，還有一種能夠在O(1)空間複雜度完成的解法，有些難想，詳情能夠參考https://leetcode.com/problems/longest-valid-parentheses/solution/#