2017-09-21 LeetCode_032 Longest Valid Parentheses

.32. Longest Valid Parentheses

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.
solution:

#include <stack>

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class Solution {

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public:

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   int longestValidParentheses(string s) {

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       int ans = 0;

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       stack<int> used; used.push(-1);

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       for (int i = 0; i <= s.length(); i++) {

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           ans = ans > i-used.top()-1 ? ans : i-used.top()-1;

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           if (i == s.length()) break;

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if (used.top() >= 0 && s[used.top()] == '(' && s[i] == ')') used.pop();

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           else used.push(i);

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}

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return ans;

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   }

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};