leetcode 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 get(1,n);
}

public ArrayList<TreeNode> get(int s,int e){
ArrayList<TreeNode> res= new ArrayList<TreeNode>();
if(s>e) {res.add(null);return res;}
for(int i=s;i<=e;i++){
ArrayList<TreeNode> l=get(s,i-1);
ArrayList<TreeNode> r=get(i+1,e);

for(int m=0;m<l.size();m++){
for(int n=0;n<r.size();n++){
TreeNode c= new TreeNode(i);
c.left=l.get(m);
c.right=r.get(n);
res.add(c);
}}
}
return res;
}
}