Unique Binary Search Trees

1、Unique Binary Search Trees

class Solution {
public:
    int numTrees(int n) {
        if(n < 1)
             return 1;
        vector<int> res(n+1, 0);
        res[0] = 1;
        res[1] = 1;
        for(int i=2; i<=n; i++)
        {
            for(int j=0; j<i; j++)
            {
                res[i] += res[j]*res[i-1-j];
            }
        }
        return res[n];
    }
};