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;

    }

};