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

 

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

 

This method to this problem is similar to Unique Binary Search Trees.

 

public class Solution {
    public ArrayList<TreeNode> generateTrees(int n) {
         return treeHelp(1, n);
    }
	public ArrayList<TreeNode> treeHelp(int l, int n){
		ArrayList<TreeNode> result = new ArrayList<TreeNode>();
		if(l > n){ // no TreeNode can be generated 
			result.add(null);
			return result;
		}
		else{
			for(int i = l; i <= n; ++i){
				ArrayList<TreeNode> leftNode = treeHelp(l, i - 1);
				ArrayList<TreeNode> rightNode = treeHelp(i + 1, n);
				for(int j = 0; j < leftNode.size(); ++j){
					for(int m = 0; m < rightNode.size(); ++m){
						TreeNode root = new TreeNode(i);
						root.left = leftNode.get(j);
						root.right = rightNode.get(m);
						result.add(root);
					}
				}
			}		
		}
		return result;
    }
}