忘记root密码怎么办

$ mysql -u root
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 4
Server version: 5.7.17

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mysql> select * from user;
ERROR 1046 (3D000): No database selected
mysql> use mysql
ERROR 1820 (HY000): You must reset your password using ALTER USER statement before executing this statement.
mysql> use mysql;
ERROR 1820 (HY000): You must reset your password using ALTER USER statement before executing this statement.
mysql> set password = PASSWORD('12345')
-> ;
Query OK, 0 rows affected, 1 warning (0.01 sec)

mysql> alter user 'root'@'localhost' PASSWORD EXPIRE NEVER
-> ;
Query OK, 0 rows affected (0.00 sec)

mysql> FLUSH PRIVILEGES;
Query OK, 0 rows affected (0.01 sec)

mysql>