Unique Binary Search Trees

 1 class Solution {
 2 public:
 3     int numTrees(int n) {
 4         if(n < 1)
 5              return 1;
 6         vector<int> res(n+1, 0);
 7         res[0] = 1;
 8         res[1] = 1;
 9         for(int i=2; i<=n; i++)
10         {
11             for(int j=0; j<i; j++)
12             {
13                 res[i] += res[j]*res[i-1-j];
14             }
15         }
16         return res[n];
17     }
18 };