Problem
Write a program for user registration:
• First the program tells the user "enter 1 to register, enter 2 to sign in, enter 3 to quit"
• After the user enter 1, the program ask the user to input username and password, the username can not be the same as any existing ones.
• After registration, tell user "Congratulations! You have an account with us" direct user back to the starting point.
• If the user enter 2, hint the user either "Wrong password" or "Wrong username" until user enters a correct combination
• Display "Welcome back to your account!" after user entered the correct user name and password.