leetcode Longest Valid Parentheses

class Solution {  
 public:  
  int longestValidParentheses(string s) {  
    // Note: The Solution object is instantiated only once and is reused by each test case.  
    stack<int> sta;  
    int i = 0, j = 0, maxV = 0, count = 0;  
    for (i = 0; i < s.length(); ++i) {  
      if (s[i] == '(')  
        sta.push(i);  
      else {  
        if (!sta.empty() && s[sta.top()] == '(') {  
          sta.pop();  
          if (sta.empty())  
            count = i + 1;  
          else  
            count = i - sta.top();  
          if (count > maxV)  
            maxV = count;  
        }  
        else  
          sta.push(i);  
      }  
    }  
    return maxV;  
  }  
};