public boolean isBalanced(TreeNode root) {        int res = helper(root);        if (res<0) return false;        return true;    }    public int helper(TreeNode root)    {        if (root==null) return 0;        //从底下开始判断是否平衡树        //两个变量如果是-1就代表是不平衡        int ld = helper(root.left);        int rd = helper(root.right);        //三种情况就不平衡：左右子树不平衡，本节点不平衡        if (ld==-1||rd==-1||Math.abs(ld-rd)>1)            return -1;        else if (ld>rd) return ld+1;        else return rd+1;    }