Leetcode: Unique Binary Search Tree 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

public ArrayList<TreeNode> generateTrees(int n) {// Start typing your Java solution below// DO NOT write main() functionArrayList<TreeNode> res = new ArrayList<TreeNode>();generate(res, 1, n);return res;}public void generate(ArrayList<TreeNode> res, int start, int end){if(start > end){res.add(null);// reason of adding NULL is because OJ uses this format to store treesreturn;}for(int i = start; i <= end; i++){ArrayList<TreeNode> left = new ArrayList<TreeNode>();ArrayList<TreeNode> right = new ArrayList<TreeNode>();generate(left, start, i - 1);generate(right, i + 1, end);// i is current tree node, left stores left child node, right stores right child node// all nodes in left is smaller than i, all nodes in right is larger than i// if n == 2;// 1st iteration: left null, right 2;// 2nd iteration: left 1, right null;for(int j = 0; j < left.size(); j++){for(int k = 0; k < right.size(); k++){TreeNode root = new TreeNode(i);root.left = left.get(j);root.right = right.get(k);res.add(root);}}}}