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.

/**

 * 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 ArrayList<TreeNode> generateTrees(int n) {

        return genTrees(1, n);

    }

    

    public ArrayList<TreeNode> genTrees(int a, int b) {

        ArrayList<TreeNode> trees = new ArrayList<TreeNode>();

        if(a>b) {

            trees.add(null);

        } else if(a==b) {

            trees.add(new TreeNode(a));

        } else {

            for(int i=a;i<=b;i++) {

                ArrayList<TreeNode> lefts = genTrees(a, i-1);

                ArrayList<TreeNode> rights = genTrees(i+1, b);

                for(TreeNode l : lefts) {

                    for(TreeNode r : rights) {

                        TreeNode tree = new TreeNode(i);

                        tree.left = l;

                        tree.right = r;

                        trees.add(tree);

                    }

                }

            }

        }

        return trees;

    }

}