[LeetCode] Longest Valid Parentheses

[Problem]

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

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

Another example is ")()())", where the longestvalid parentheses substring is "()()", which haslength = 4.

[Analysis]
注意是最长的合法圆括号，()(() 存在合法圆括号有4个，前面2个，后面2个，但是连续最长的是2个。同样，)()())()()(存在合法圆括号有8个，下标为1,2,3,4,6,7,8,9，但是连续最长的是1,2,3,4或者6,7,8,9，最长的是4个。
开始没看清楚题，以为是求合法圆括号总个数，一直WA。

[Solution]

class Solution {
public:
    int longestValidParentheses(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
       
        int len = s.length();
        stack<int> myStack;
        for(int i = 0; i < len; ++i){
            if(s[i] == '('){
                myStack.push(i);
            }
            else if(s[i] == ')'){
                if(myStack.size() <= 0 || s[myStack.top()] != '('){
                    myStack.push(i);
                }
                else{
                    myStack.pop();
                }
            }
        }
       
        // all of the parentheses are valid
        if(myStack.empty()){
            return len;
        }
       
        // search the longest valid parentheses
        int max = len-1-myStack.top();
        while(!myStack.empty()){
            int top = myStack.top();
            myStack.pop();
            if(!myStack.empty()){
                int len = top - myStack.top() - 1;
                max = max > len ? max : len;
            }
            else{
                max = max > top ? max : top;
            }
        }
        return max;
    }
};

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