LeetCode 902 Numbers At Most N Given Digit Set - Problem Statement

Given an array of digits (like $[1, 3, 5, 7]$) and an integer $n$, return the count of positive integers that can be formed using only those digits and are at most $n$. For digits $= [1, 3, 5, 7]$ and $n = 100$, you can form: $1, 3, 5, 7$ (one-digit), $11, 13, 15, ..., 77$ (two-digit), but nothing three-digit since $111 > 100$.

Answer: $20$. This is a clean introduction to digit DP. The only constraint is which digits you can use.