Unique Binary Search Trees II

Source

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

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

Java

/**
 * 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) {
        if(n==0) return new ArrayList<TreeNode>();
        return generateTrees(1, n);
    }

    public List<TreeNode> generateTrees(int start, int end) {
        List<TreeNode> list = new LinkedList<>();

        if (start > end) {
            list.add(null);
            return list;
        }

        for (int i = start; i <= end; i++) {
            List<TreeNode> lefts = generateTrees(start, i - 1);
            List<TreeNode> rights = generateTrees(i + 1, end);
            for (TreeNode left : lefts) {
                for (TreeNode right : rights) {
                    TreeNode node = new TreeNode(i);
                    node.left = left;
                    node.right = right;
                    list.add(node);
                }
            }
        }
        return list;
    }
}