Unique Binary Search Trees II

Given n, generate all structurally unique BST's (binary search trees) that store values 1...n.

For example,
Given n = 3, your program should return all 5 unique BST's shown below.

   1         3     3      2      1
    \       /     /      / \      \
     3     2     1      1   3      2
    /     /       \                 \
   2     1         2                 3

public class Solution {
    public ArrayList<TreeNode> generateTrees(int n) {
        return builder(1,n);
    }
    public ArrayList<TreeNode> builder(int start,int end){
        ArrayList<TreeNode> sub = new ArrayList<TreeNode>();
        if(start>end){
            sub.add(null);
            return sub;
        }
        for(int i=start;i<=end;i++){
            ArrayList<TreeNode> left = builder(start,i-1);
            ArrayList<TreeNode> right = builder(i+1,end);
            for(int j=0;j<left.size();j++){
                for(int k=0;k<right.size();k++){
                    TreeNode root = new TreeNode(i);
                    root.left = left.get(j);
                    root.right = right.get(k);
                    sub.add(root);
                }
            }
        }
        return sub;
    }
}