Leetcode -- Longest Valid Parentheses

class Solution {  
 public:  
   int longestValidParentheses(string s) {  
     int n = s.length();  
     vector<int> stack;  
     int maxLength = 0;  
     int last = -1;  
     for(int i = 0; i < n; ++i) {  
       if(s[i] == '(') {  
         stack.emplace_back(i);  
       } else {  
         if(stack.empty()) {  
           last = i;  
         } else {  
           stack.pop_back();  
           if(stack.empty()) {  
             maxLength = max(maxLength, i - last);  
           } else {  
             maxLength = max(maxLength, i - stack[stack.size() - 1]);  
           }  
         }  
       }  
     }  
    return maxLength;  
   }  
 };