判断是否为AVL树

时间复杂度：O(n)

// 判断是否为AVL树
public int isAVL(TreeNode node) {
    if (node == null) {
        return 0;
    }
    int left = isAVL(node.left);
    if (left == -1) {
        return -1;
    }
    int right = isAVL(node.right);
    if (right == -1) {
        return -1;
    }
    int diff = left - right;
    if (Math.abs(diff) > 1) {
        return -1;
    } else {
        return Math.max(left, right) + 1;
    }
}
public static boolean isBalanced(TreeNode root) {
    if (isAVL(root) == -1) {
        return false;
    } else {
        return true;
    } 
}