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Diffstat (limited to 'num_music_playlists.c')
-rw-r--r-- | num_music_playlists.c | 39 |
1 files changed, 39 insertions, 0 deletions
diff --git a/num_music_playlists.c b/num_music_playlists.c new file mode 100644 index 0000000..c06ee28 --- /dev/null +++ b/num_music_playlists.c @@ -0,0 +1,39 @@ +// 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 +} |