[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.

 1 struct Node
 2 {
 3     char c;
 4     int index;
 5     Node(){}
 6     Node(char _c, int idx):c(_c), index(idx){}
 7 };
 8 
 9 class Solution {
10 public:
11     int longestValidParentheses(string s) {
12         // Start typing your C/C++ solution below
13         // DO NOT write int main() function
14         stack<Node> st;
15         st.push(Node(')', -1));
16         int ret = 0;
17         for(int i = 0; i < s.size(); i++)
18         {
19             char c = s[i];
20             if (c == '(')
21                 st.push(Node(c, i));
22             else
23             {
24                 Node node = st.top();
25                 if (node.c == '(')
26                 {
27                     st.pop();
28                     ret = max(ret, i - st.top().index);
29                 }
30                 else
31                     st.push(Node(c, i));                   
32             }
33         }
34         
35         return ret;
36     }
37 };