Add solution #920

Author: JoshCrozier

README.md

@@ -729,6 +729,7 @@
 916|[Word Subsets](./solutions/0916-word-subsets.js)|Medium|
 918|[Maximum Sum Circular Subarray](./solutions/0918-maximum-sum-circular-subarray.js)|Medium|
 919|[Complete Binary Tree Inserter](./solutions/0919-complete-binary-tree-inserter.js)|Medium|
+920|[Number of Music Playlists](./solutions/0920-number-of-music-playlists.js)|Hard|
 922|[Sort Array By Parity II](./solutions/0922-sort-array-by-parity-ii.js)|Easy|
 925|[Long Pressed Name](./solutions/0925-long-pressed-name.js)|Easy|
 926|[Flip String to Monotone Increasing](./solutions/0926-flip-string-to-monotone-increasing.js)|Medium|

solutions/0920-number-of-music-playlists.js

@@ -0,0 +1,37 @@
+/**
+ * 920. Number of Music Playlists
+ * https://leetcode.com/problems/number-of-music-playlists/
+ * Difficulty: Hard
+ *
+ * Your music player contains n different songs. You want to listen to goal songs (not necessarily
+ * different) during your trip. To avoid boredom, you will create a playlist so that:
+ * - Every song is played at least once.
+ * - A song can only be played again only if k other songs have been played.
+ *
+ * Given n, goal, and k, return the number of possible playlists that you can create. Since the
+ * answer can be very large, return it modulo 109 + 7.
+ */
+
+/**
+ * @param {number} n
+ * @param {number} goal
+ * @param {number} k
+ * @return {number}
+ */
+var numMusicPlaylists = function(n, goal, k) {
+  const MOD = 1e9 + 7;
+  const dp = new Array(goal + 1).fill().map(() => new Array(n + 1).fill(0));
+  dp[0][0] = 1;
+
+  for (let songs = 1; songs <= goal; songs++) {
+    for (let uniques = 1; uniques <= Math.min(songs, n); uniques++) {
+      dp[songs][uniques] = dp[songs - 1][uniques - 1] * (n - (uniques - 1));
+      if (uniques > k) {
+        dp[songs][uniques] += dp[songs - 1][uniques] * (uniques - k);
+      }
+      dp[songs][uniques] %= MOD;
+    }
+  }
+
+  return dp[goal][n];
+};