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;
}
}