【ATT】Longest Valid Parentheses

Q: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.

A: 遇到valid括号问题，一般使用栈来解决。

遇到左括号，压入栈内；

遇到右括号，如果栈不为空，pop栈（即增加一对有效括号），统计当前有效括号子串长度，同时更新maxlen。统计当前有效括号子串长度，有两种情况：一种是当前栈不为空，curlen = i - stack.top();栈为空时 curlen = i - last;

　　　　　　如果栈为空，说明此时的“（”是最近一个无效的'('

    int longestValidParentheses(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        stack<int> st;
        
        int last = -1;
        int maxlen = 0;
        
        for(int i=0;i<s.size();i++)
        {
            if(s[i]=='(')
                st.push(i);
            else{    
                if(!st.empty())
                {
                    st.pop();
                    if(st.empty())
                        maxlen = max(maxlen,i-last);
                    else
                        maxlen = max(maxlen,i - st.top());
                }else
                    last = i;
            }
        }
        
        return maxlen;
        
    }