Kata

You need to write a password generator that meets the following criteria:

• 6 - 20 characters long
• contains at least one lowercase letter
• contains at least one uppercase letter
• contains at least one number
• contains only alphanumeric characters (no special characters)

Return the random password as a string.

Note: "randomness" is checked by counting the characters used in the generated passwords - all characters should have less than 50% occurance. Based on extensive tests, the normal rate is around 35%.