Leetcode 920. Number of Music Playlists

Count the number of length-goal sequences using n distinct songs such that every song appears at least once and a song can be replayed only after k other songs have been played, returning the result modulo 1e9+7. This is a combinatorics/DP problem (inclusion–exclusion or recurrence) to account for permutations and the cooldown constraint with n ≤ goal ≤ 100.