LeetCode -- Longest Valid Parentheses

题目描述：


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.






思路：
1.遍历s[i] ,i∈[0,n)并使用stack存当前索引i
2.如果s[i] 为 ')' 且stack不为空 且s[stack.Peek()] 为'('
:stack弹出
如果stack为空 , max = i + 1
否则，max = Max(max,i-stack.Peek())
  否则(即s[i]为'(')，直接将s[i]入栈




实现代码：

public class Solution {    public int LongestValidParentheses(string s) {        int max = 0;        var stack = new Stack<int>();        for (int i = 0; i < s.Length; i++) {            if (s[i] == ')' && stack.Count > 0 && s[stack.Peek()] == '(') {                stack.Pop();                if (stack.Count == 0){    max = i + 1;    }                else{    max = Math.Max(max, i - stack.Peek());    }            } else {                stack.Push(i);            }        }            return max;    }}