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;
    }
}