<p>Your music player contains <code>n</code> different songs. You want to listen to <code>goal</code> songs (not necessarily different) during your trip. To avoid boredom, you will create a playlist so that:</p>

<ul>
	<li>Every song is played <strong>at least once</strong>.</li>
	<li>A song can only be played again only if <code>k</code> other songs have been played.</li>
</ul>

<p>Given <code>n</code>, <code>goal</code>, and <code>k</code>, return <em>the number of possible playlists that you can create</em>. Since the answer can be very large, return it <strong>modulo</strong> <code>10<sup>9</sup> + 7</code>.</p>
<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> n = 3, goal = 3, k = 1
<strong>Output:</strong> 6
<strong>Explanation:</strong> There are 6 possible playlists: [1, 2, 3], [1, 3, 2], [2, 1, 3], [2, 3, 1], [3, 1, 2], and [3, 2, 1].
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> n = 2, goal = 3, k = 0
<strong>Output:</strong> 6
<strong>Explanation:</strong> There are 6 possible playlists: [1, 1, 2], [1, 2, 1], [2, 1, 1], [2, 2, 1], [2, 1, 2], and [1, 2, 2].
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> n = 2, goal = 3, k = 1
<strong>Output:</strong> 2
<strong>Explanation:</strong> There are 2 possible playlists: [1, 2, 1] and [2, 1, 2].
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>0 &lt;= k &lt; n &lt;= goal &lt;= 100</code></li>
</ul>

class Solution {
public:
    const int MOD = 1e9 + 7;
    int N, GOAL, K;

    int solve(int countSong, int countUnique, vector<vector<int>> &dp) {
        if (countSong == GOAL) {
            return countUnique == N ? 1 : 0;
        }

        if (dp[countSong][countUnique] != -1) {
            return dp[countSong][countUnique];
        }

        long long result = 0;
        
        // if play new song
        if (N > countUnique) {
            result += (long long)(N - countUnique) * solve(countSong + 1, countUnique + 1, dp) % MOD;
            result %= MOD; 
        }
        
        // replay old song
        if (countUnique > K) {
            result += (long long)(countUnique - K) * solve(countSong + 1, countUnique, dp) % MOD;
            result %= MOD; 
        }

        return dp[countSong][countUnique] = result;
    }

    int numMusicPlaylists(int n, int goal, int k) {
        N = n;
        GOAL = goal;
        K = k;
        vector<vector<int>> dp(101, vector<int>(101, -1));
        return solve(0, 0, dp);
    }
};

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