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 binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; left = null; right = null; }
 * }
 */
public class Solution {
    public List<TreeNode> generateTrees(int m, int n){
        List<TreeNode> ans = new LinkedList<TreeNode>();
        if(m > n){
            ans.add(null);
        }
        for(int i = m; i<=n; i++){
            List<TreeNode> leftTrees = generateTrees(m, i-1);
            List<TreeNode> rightTrees = generateTrees(i+1, n);
            for(TreeNode left: leftTrees){
                for(TreeNode right: rightTrees){
                    TreeNode root = new TreeNode(i);
                    root.left = left;
                    root.right = right;
                    ans.add(root);
                }
            }
        }
        return ans;
    }
    
    public List<TreeNode> generateTrees(int n) {
        return generateTrees(1, n);
    }
}