MySQL ERROR 1698 (28000): Access denied for user ‘root‘@‘localhost‘

Question：

ufo@UPC:~$ mysql -u root -p root;
Enter password: 
ERROR 1698 (28000): Access denied for user 'root'@'localhost'

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Step 1：login

line A:

sudo gedit /etc/mysql/mysql.conf.d/mysqld.cnf

add comment [ skip-grant-tables ] 

[mysqld]
user		= mysql
bind-address		= 127.0.0.1
mysqlx-bind-address	= 127.0.0.1

skip-grant-tables

service mysql restart;

$ mysql -uroot -p;

OR 

line B:

$ sudo mysql -uroot -proot;

 The garget :

step 2：check

use mysql;

flush privileges;

select user,plugin from user;

step 3：action

ALTER USER 'root'@'%' IDENTIFIED WITH mysql_native_password BY '123456';

ALTER USER 'root'@'%' IDENTIFIED BY '123456' PASSWORD EXPIRE NEVER;

flush privileges; 

 select user,plugin from user;

The results:

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If you choice Step1-lineA,you'll...:

sudo gedit /etc/mysql/mysql.conf.d/mysqld.cnf

 #skip-grant-tables

service mysql restart;

done!