Longest Valid Parentheses

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#include <iostream>#include <stack>#include <string>#include <algorithm>using namespace std;/**Longest Valid ParenthesesGiven 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.思路：1.设置一个栈来保存可以待匹配的‘（’，由于只有'（' 和' ）' ，所以可以简单的用if -else表示2.如果栈是空的（匹配完了上一组（）情况，或者之前根本没有可以匹配的例如多个“））” ），这时设置一个last来记录最近未匹配的索引值3.如果栈非空，则计算i和sta.top()的差值找到最大长度*/class Solution {public:    int longestValidParentheses(string s) {        if(s == "")            return 0;        int maxvalid = 0;        int last = -1,i ;        stack<int> sta;        for(i=0;i < s.length();i++){            if(s[i] == '(')               sta.push(i);            else{                if(sta.empty())                    last = i;                else{                    sta.pop();                    if(sta.empty())                        maxvalid = max(maxvalid, i-last);                    else                        maxvalid = max(maxvalid,i-sta.top());                }            }        }        return maxvalid;    }};int main(){    string s = ")()()((()()()()()";    Solution sol;    int r = sol.longestValidParentheses(s);    cout << r << endl;    return 0;}
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