给定一个正整数n，返回从1到n构成的所有的BST

public class C3 {

    public static void main(String[] args) {

        ArrayList<TreeNode> res = generateTrees(5);

        System.out.println(res);

    }

    public static ArrayList<TreeNode> generateTrees(int n) {

        if(n == 0){

            return  new ArrayList<TreeNode>();

        }

        return generateTrees(1, n);//从1作为root开始，到n作为root结束

    }

    private static ArrayList<TreeNode> generateTrees(int left, int right){

        ArrayList<TreeNode> res = new ArrayList<TreeNode>();

        if (left > right){

            res.add(null);

            return res;

        }

        for (int i = left; i <= right; i++){

            ArrayList<TreeNode> lefts = generateTrees(left, i-1);     //以i作为根节点，左子树由[1,i-1]构成

            ArrayList<TreeNode> rights = generateTrees(i+1, right);   //右子树由[i+1, n]构成

            for (int j = 0; j < lefts.size(); j++){

                for (int k = 0; k < rights.size(); k++){

                    TreeNode root = new TreeNode(i);

                    root.left = lefts.get(j);

                    root.right = rights.get(k);

                    res.add(root);//存储所有可能行

                }

            }

        }

        return  res;

    }

    public static class TreeNode {

        TreeNode left;

        TreeNode right;

        int val;

        public TreeNode () {}

        public TreeNode (int val) {

            this.val = val;

        }

    }

}

给定一个正整数n，返回从1到n构成的所有的BST的相关教程结束。