No.96Unique Binary Search Trees

//Given n, how many structurally unique BST's (binary search trees) that store values 1 ... n?

//solution

 int numTrees(int n) 
    {
        int res=0;
        if(n==0||n==1)
            return 1;
        else if(n==2)
            return 2;
        else
            for(int i=1;i<=n;i++)
                res+=numTrees(i-1)*numTrees(n-i);
                
        return res;
    }