【leetcode】Longest Valid Parentheses

Question：

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.

Anwser 1 ：      

class Solution {public:    int longestValidParentheses(string s) {        // Start typing your C/C++ solution below        // DO NOT write int main() function                stack<int> S;                for(int i = 0; i < s.size(); ++i){      // "("            if (s[i] == '(') {                S.push(i);            } else if (!S.empty()){     //')'                s[i] = 'k';                s[S.top()] = 'k';                S.pop();            }        }        // example: "(()" = "(kk";  "(()()" = "(kkkk"                int maxLength = 0;        int length = 0;        for(int i = 0; i < s.size(); ++i){  // count of "k"            if (s[i] =='k'){                ++length;                                if (maxLength < length) {                    maxLength = length;                }            } else {                length = 0;            }        }                return maxLength;    }};

Anwser 2 ：    

class Solution {public:    struct node {        char c;        int idx;        node() {}        node(char _c, int _idx): c(_c), idx(_idx) {}            };        int longestValidParentheses(string s) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        stack<node> st;        st.push(node(')', -1));        int res = 0;                for (int i = 0; i < s.size(); ++i)        {            char c = s[i];            if (c == '(') {                st.push(node(c, i));            } else {                node top = st.top();                if (top.c == '(') {                    st.pop();                    res = max(res, i - st.top().idx);   //  count or length                } else {                    st.push(node(c, i));                }            }        }                return res;    }};