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> allTrees = new LinkedList<TreeNode>();

        if (start > end) {

            allTrees.add(null);

            return allTrees;

        }

        // 枚举可行根节点

        for (int i = start; i <= end; i++) {

            // 获得所有可行的左子树集合

            List<TreeNode> leftTrees = generateTrees(start, i - 1);

            // 获得所有可行的右子树集合

            List<TreeNode> rightTrees = generateTrees(i + 1, end);

            // 从左子树集合中选出一棵左子树，从右子树集合中选出一棵右子树，拼接到根节点上

            for (TreeNode left : leftTrees) {

                for (TreeNode right : rightTrees) {

                    TreeNode currTree = new TreeNode(i);

                    currTree.left = left;

                    currTree.right = right;

                    allTrees.add(currTree);

                }

            }

        }

        return allTrees;

    }

}