How to Solve Cracking the Safe Problem

There is a safe protected by a password. The password is a sequence of n digits where each digit can be in the range [0, k - 1]. The safe has a peculiar way of checking the password. When you enter in a sequence, it checks the most recent n digits that were entered each time you type a digit. For example, the correct password is "345" and you enter in "012345": After typing 0, the most recent 3 digits is "0", which is incorrect. After typing 1, the most recent 3 digits is "01", which is incorrect. After typing 2, the most recent 3 digits is "012", which is incorrect. After typing 3, the most recent 3 digits is "123", which is incorrect. After typing 4, the most recent 3 digits is "234", which is incorrect. After typing 5, the most recent 3 digits is "345", which is correct and the safe unlocks. Return any string of minimum length that will unlock the safe at some point of entering it.