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

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<TreeNode> generateTrees(int n) {
        return generateTrees(1, n);
    }

	private List<TreeNode> generateTrees(int left, int right) {
		List<TreeNode> res = new ArrayList<>();
		if (left > right) {
			res.add(null);
			return res;
		}
		for (int i = left; i <= right; i++) {
			List<TreeNode> lefts = generateTrees(left, i-1);
			List<TreeNode> rights = generateTrees(i+1, right);
			for (int j = 0; j < lefts.size(); j++) {
				for (int k = 0; k < rights.size(); k++) {
					TreeNode root = new TreeNode(i);
					root.left = lefts.get(j);
					root.right = rights.get(k);
					res.add(root);
				}
			}
		}
		return res;
	}
}