[LeetCode] 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.

struct Node
{
    char c;
    int index;
    Node(){}
    Node(char _c, int idx):c(_c), index(idx){}
};

class Solution {
public:
    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 ret = 0;
        for(int i = 0; i < s.size(); i++)
        {
            char c = s[i];
            if (c == '(')
                st.push(Node(c, i));
            else
            {
                Node node = st.top();
                if (node.c == '(')
                {
                    st.pop();
                    ret = max(ret, i - st.top().index);
                }
                else
                    st.push(Node(c, i));                   
            }
        }
        
        return ret;
    }
};