0032.Longest Valid Parentheses

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1032.Longest Valid Parentheses
Given a string containing just the characters '(' and ')', find the length of the longest valid (well-formed) parentheses substring.

Example 1:

Input: "(()"
Output: 2
Explanation: The longest valid parentheses substring is "()"
Example 2:

Input: ")()())"
Output: 4
Explanation: The longest valid parentheses substring is "()()"

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class Solution {
public:
    int longestValidParentheses(string s) {
        stack<int> st;
        int i, size = s.size(), lastL = 0, max = 0;
        for(i = 0; i < size; i++){
            if(s[i] == '(')
                st.push(i); //存左括号的下标
            else{
                if(st.empty()) lastL = i+1; // 右括号多了，这种情况比较简单，直接开始新的计数，即重置lastL
                else{
                    st.pop();
                    if(st.empty()) // 左括号刚好用完
                        max = (i-lastL+1) > max? (i-lastL+1): max;   
                    else // 左括号没有用完，这时候不能重置lastL，因为后面可能有右括号和多余的左括号配对
                        max = (i-st.top()) > max? (i-st.top()): max;
                }
            }
        }
        return max;
    }
};