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
 

 /**
  * 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);
     }
     public ArrayList<TreeNode> generateTrees(int a, int b){
         ArrayList<TreeNode> res = new ArrayList<TreeNode>();
         if(a>b) res.add(null);
         for(int i=a;i<=b;i++){
             ArrayList<TreeNode> temp1 = generateTrees(a,i-1);
             ArrayList<TreeNode> temp2 = generateTrees(i+1,b);
             for(TreeNode n:temp1)
                 for(TreeNode m:temp2){
                     TreeNode temp= new TreeNode(i);
                     temp.left=n;
                     temp.right=m;
                     res.add(temp);
                 }
         }
         return res;
     }
 }