[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.

#include <stack>
using namespace std;

class Solution {
public:
int longestValidParentheses(string s) {
// Start typing your C/C++ solution below
// DO NOT write int main() function

int maxLength = 0;
int size = s.size();
stack<int> left;
if(size<=0) return 0;
int top = size;

for(int i = 0; i<size; i++)
{
if(s[i]=='(')
{
left.push(i);
}
else
{
if(left.empty())
{
top = i + 1;//指向下一个为")"的index
}
else
{
int tmp = left.top();
top = min(top,tmp);
left.pop();
if(left.empty()) maxLength = max(i-top+1,maxLength);
else maxLength = max(i-left.top(),maxLength); //注意：为什么是left.top()而不是tmp-1呢？考虑"(()()"的情况。
}
}
}

return maxLength;
}
};