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;
        }
    }
}