LeetCode:Longest Valid Parentheses

Longest Valid Parentheses

Total Accepted: 63715 Total Submissions: 282480 Difficulty: Hard

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

For "(()", the longest valid parentheses substring is "()", which has length = 2.

Another example is ")()())", where the longest valid parentheses substring is "()()", which has length = 4.

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 (E) Valid Parentheses

思路：

使用stack来保存未匹配的“括号”的下标，然后求下标之间的最大差值。

c++ code:

class Solution {public:    int longestValidParentheses(string s) {        int n = s.length();        stack<int> st;        for (int i = 0; i < n; i++) {            if (s[i] == '(') st.push(i);            else {                if (!st.empty()) {                    if (s[st.top()] == '(') st.pop();                    else st.push(i);                }                else st.push(i);            }        }                int a = n, b = 0;        int ans = 0;        while(!st.empty()) {            b = st.top();            st.pop();            ans = max(ans, a-b-1);            a = b;        }        ans = max(ans, a);        return ans;    }};