95. Unique Binary Search Trees II

Given an integer 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

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<TreeNode> generateTrees(int n) {
        if(n < 1)
            return new ArrayList<>();
        return generateSubTree(1,n);
    }
    public List<TreeNode> generateSubTree(int left, int right){
        List<TreeNode> res = new ArrayList<TreeNode>();
        if(left > right){
            res.add(null);
            return res;
        }
        for(int i = left; i <= right; i++){
             List<TreeNode> leftTree = generateSubTree(left, i-1);
             List<TreeNode> rightTree = generateSubTree(i+1, right);
             for(int j = 0 ; j < leftTree.size(); j ++){
                 for(int k = 0; k < rightTree.size(); k++){
                     TreeNode root = new TreeNode(i);
                     root.left = leftTree.get(j);
                     root.right = rightTree.get(k);
                     res.add(root);
                 }
             }
        }
        return res;
            
    }
}