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;
        //從底下開始判斷是否平衡樹
        //兩個變量如果是-1就代表是不平衡
        int ld = helper(root.left);
        
 
int rd = helper(root.right);
        //三種情況就不平衡：左右子樹不平衡，本節點不平衡
        if (ld==-1||rd==-1||Math.abs(ld-rd)>1)
            return -1;
        else if (ld>rd) return ld+1;
        else return rd+1;
    }

[leetcode]110BalancedBinaryTree平衡二叉樹