LeetCode: Longest Valid Parentheses

用dfs large超时，看了网上答案用stack，其实一开始也想到了，只是没预料到stack比dfs更优越

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

 C#

public class Solution {
    public class node {
        public char c;
        public int index;
        public node (char _c, int _index) {
            c = _c;
            index = _index;
        }
    };
    public int LongestValidParentheses(string s) {
        if (s.Length <= 1) return 0;
        Stack<node> S = new Stack<node>();
        int maxLen = 0;
        S.Push(new node(')', -1));
        int lastLen = 0;
        for (int i = 0; i < s.Length; i++) {
            if (s[i] == '(') S.Push(new node('(', i));
            else {
                node tmp = S.Peek();
                if (tmp.c == '(') {
                    S.Pop();
                    maxLen = Math.Max(maxLen, i - S.Peek().index);
                }
                else S.Push(new node(')', i));
            }
        }
        return maxLen;
    }
}