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

Use recursion to get all possible roots, right nodes and left nodes. The returned roots are used as left or right nodes in next recursion.

/**
* 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) {
return generate(1, n);
}

private ArrayList<TreeNode> generate(int start, int end) {
ArrayList<TreeNode> res = new ArrayList<TreeNode>();

if (start > end) {
res.add(null);
return res;
}

for (int i = start; i <= end; i++) {
ArrayList<TreeNode> left = generate(start, i - 1);
ArrayList<TreeNode> right = generate(i + 1, end);
for (TreeNode l : left) {
for (TreeNode r : right) {
TreeNode root = new TreeNode(i);
root.left = l;
root.right = r;
res.add(root);
}
}
}

return res;
}
}