/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<TreeNode> generateTrees(int n) {
        if (n==0)
            return new LinkedList<TreeNode>();
        return generateTrees(1,n);
    }
    public List<TreeNode> generateTrees(int start, int end) {
        List<TreeNode> res = new LinkedList<TreeNode>();
        if (start > end) {
            res.add(null);
            return res;
        }
        for (int i = start; i <= end; i++) {
            List<TreeNode> subLeftTree = generateTrees(start, i-1);
            List<TreeNode> subRightTree = generateTrees(i+1, end);
            for (TreeNode left: subLeftTree) {
                for (TreeNode right: subRightTree) {
                    TreeNode node = new TreeNode(i);
                    node.left = left;
                    node.right = right;
                    res.add(node);
                }
            }
        }
        return res;
    }
}