[leetcode]longest Valid Parentheses(!!)

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