[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

[Solution].java

/**
 * 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) {
        // Start typing your Java solution below
        // DO NOT write main() function
        return generateTrees(1, n);
    }

    private ArrayList<TreeNode> generateTrees(int begin, int end){
        ArrayList<TreeNode> result = new ArrayList<TreeNode>();
        if(begin > end) result.add(null);
        else{
            for(int i = begin; i <= end; i++){
                ArrayList<TreeNode> left = generateTrees(begin, i - 1);
                ArrayList<TreeNode> right = generateTrees(i + 1, end);
                for(int j = 0; j < left.size(); j++){
                    for(int k = 0; k < right.size(); k++){
                        TreeNode node = new TreeNode(i);
                        node.left = left.get(j);
                        node.right = right.get(k);
                        result.add(node);
                    }
                }
            }
        }
        return result;
    }
}