Merge pull request #2769 from sumit16sharma/main

Created 0920-number-of-music-playlists.java

Author: neetcode-gh

java/0920-number-of-music-playlists.java

@@ -0,0 +1,58 @@
+class Solution {
+    public int numMusicPlaylists(int n, int goal, int k) {
+        map = new HashMap<>();
+        return (int)findPlaylists(0, goal, n, k);
+    }
+
+    HashMap<Pair, Integer> map;
+    int mod = 1000000007;
+
+    public long findPlaylists(int old_songs, int goal, int n, int k) {
+        if(goal == 0 && old_songs == n) {
+            return 1;
+        }
+
+        if(goal == 0 || old_songs > n) {
+            return 0;
+        }
+
+        if(map.containsKey(new Pair(old_songs, goal))) {
+            return map.get(new Pair(old_songs, goal));
+        }
+
+        // chossing an new_song
+        long res = ((n - old_songs) * (findPlaylists(old_songs + 1, goal - 1, n, k) % mod)) % mod;
+
+        // choose an old_songs
+        if(old_songs > k) {
+            res = (res + ((old_songs - k) * (findPlaylists(old_songs, goal - 1, n, k)) % mod) % mod) % mod;
+        }
+
+        map.put(new Pair(old_songs, goal), (int)res);
+
+        return res;
+    }
+
+    class Pair {
+        int a;
+        int b;
+
+        public Pair(int a, int b) {
+            this.a = a;
+            this.b = b;
+        }
+
+        @Override
+        public boolean equals(Object obj) {
+            if (this == obj) return true;
+            if (obj == null || getClass() != obj.getClass()) return false;
+            Pair pair = (Pair) obj;
+            return a == pair.a && b == pair.b;
+        }
+
+        @Override
+        public int hashCode() {
+            return 31 * a + b;
+        }
+    }
+}