LeetCode: 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.

class Solution {public:    int longestValidParentheses(string s) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        int nSize = s.size();        if (nSize <= 1)            return 0;                    stack<int> pare;        int longest = 0;        int top = nSize;        for (int i = 0; i < nSize; ++i)        {            if (s[i] == '(')                pare.push(i);            else if(!pare.empty())            {                int tmp = pare.top();                pare.pop();                top = top < tmp ? top : tmp;                if (pare.empty())                    longest = longest > i - top + 1 ? longest : i - top + 1;                else                    longest = longest > i - pare.top() ? longest : i - pare.top();            }            else                top = i + 1;        }                return longest;    }};