Question 4
A number is self-descriptive if its nth digit counts the occurrences of digit n−1 in that number. For example 21200 is self-descriptive: its initial 2 counts two zeros, the next 1 counts one 1, the next 2 counts two 2 s, and the final two zeros indicate no 3 s or 4 s. Find all four-digit self-descriptive numbers.