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.

思路：括号匹配使用栈stack来模拟匹配的过程。使用栈来存储左括号的位置信息，失去匹配的右括号作为重新计数的分隔符。

class Solution {
public:
    int longestValidParentheses(string s) {
        stack<int> st;
        int n=s.size();
        if(n==0)
            return 0;
        int last=-1;
        int longest=0;
        for(int i=0;i<n;i++)
        {
            if(s[i]=='(')
            {
                st.push(i);
            }
            else
            {
                if(st.empty())
                    last=i;
                else
                {
                    st.pop();
                    if(st.empty())
                        longest=max(longest,i-last);
                    else
                        longest=max(longest,i-st.top());
                }
            }
        }
        return longest;
    }
};