# LeetCode 95. Unique Binary Search Trees II

Problem:

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
```

Solution:

Recursion with start and end.

```/**
* 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 == 0) return new ArrayList<TreeNode>();
return generate(1, n);
}

private List<TreeNode> generate(int s, int e) {
List<TreeNode> re = new ArrayList<TreeNode>();
if (s > e) {
return re;
}
for (int i = s; i <= e; i++) {
List<TreeNode> lefts = generate(s, i - 1);
List<TreeNode> rights = generate(i + 1, e);
for (TreeNode left: lefts) {
for (TreeNode right: rights) {
TreeNode root = new TreeNode(i);
root.left = left;
root.right = right;