Leetcode Unique Binary Search Trees II

public class Solution {
public ArrayList<TreeNode> generateTrees(int n) {
return generateTrees(1, n);
}

public ArrayList<TreeNode> generateTrees(int start, int end){
ArrayList<TreeNode> unique = new ArrayList<TreeNode>();
if(start > end){
unique.add(null);
return unique;
}
for(int i = start; i <= end; i++){
for(TreeNode left: generateTrees(start, i - 1)){
for(TreeNode right: generateTrees(i + 1, end)){
TreeNode root = new TreeNode(i);
root.left = left;
root.right = right;
unique.add(root);
}
}
}
return unique;
}
}