LeetCode 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 binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; left = null; right = null; }
 * }
 */
public class Solution {
     public List<TreeNode> generateTrees(int n) {
         return create(1, n);
     }

     List<TreeNode> create(int start,int end) {
         List<TreeNode> results = new ArrayList<TreeNode>();
         if (start > end) {
             results.add(null);
             return results;
         }

         for (int k = start; k <= end; k++) {
             List<TreeNode> left = create(start, k - 1);
             List<TreeNode> right = create(k + 1, end);
             for (int i = 0; i < left.size(); i++) {
                 for (int j = 0; j < right.size(); j++) {
                     TreeNode root = new TreeNode(k);
                     root.left = left.get(i);
                     root.right = right.get(j);
                     results.add(root);
                 }
             }
         }
         return results;
     }
}