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-rw-r--r--num_music_playlists.c39
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diff --git a/num_music_playlists.c b/num_music_playlists.c
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+// 920. Number of Music Playlists
+#include <math.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+/*
+ * 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, k', return the number of possible playlists that you can
+ * create. since the answer can be very large, return it modulo 10^9+7
+ */
+
+int numMusicPlaylists(int n, int goal, int k) {
+ const int mod = 1e9 + 7;
+ long **dp = (long **)calloc(goal + 1, sizeof(long *));
+ for (int i = 0; i < goal + 1; i++)
+ dp[i] = (long *)calloc(n + 1, sizeof(long));
+ dp[0][0] = 1;
+ for (int i = 1; i <= goal; i++)
+ for (int j = 1; j <= n; j++) {
+ dp[i][j] += (dp[i - 1][j - 1] * (n - (j - 1))) % mod;
+ dp[i][j] += (dp[i - 1][j] * (long)fmax(0, j - k)) % mod;
+ dp[i][j] %= mod;
+ }
+ int ans = dp[goal][n];
+ for (int i = 0; i < goal + 1; i++)
+ free(dp[i]);
+ free(dp);
+ return ans;
+}
+
+int main() {
+ printf("%d\n", numMusicPlaylists(3, 3, 1)); // expect: 6
+ printf("%d\n", numMusicPlaylists(2, 3, 0)); // expect: 6
+ printf("%d\n", numMusicPlaylists(2, 3, 1)); // expect: 2
+}