Leetcode -- Unique Binary Search Trees

 class Solution {  
 public:  
   int numTrees(int n) {  
     return numTreesHelper(0, n - 1);  
   }  
   int numTreesHelper(int start, int end) {  
     if(start > end) {  
       return 0;  
     }  
     if(start == end) {  
       return 1;  
     }  
     int total = 0;  
     for(int i = start; i <= end; ++i) {  
       int left = numTreesHelper(start, i - 1);  
       int right = numTreesHelper(i + 1, end);  
       if(left == 0 || right == 0) {  
         if(left == 0) {  
           total += right;  
           continue;  
         }  
         if(right == 0) {  
           total += left;  
           continue;  
         }  
       } else {  
         total += left * right;  
       }  
     }  
     return total;  
   }  
 };