Number of People Aware of a Secret LeetCode solution in C++

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static int MOD = 1e9 + 7;
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class Solution {
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 public:
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  int delay, forget;
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  vector<long> memo;
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  // Total number of people who would have found out about the secret by the nth
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  // day.
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  long dp(int n) {
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    if (!n) return 0;
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    if (memo[n] != -1) // Return cached result if exists.
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      return memo[n];
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    // Current contribution of 1 person who knows the secret
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    long result = 1;
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    for (int i = delay; i < forget; i++) // Number of people that the secret will be forwarded to
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      if (n - i >= 0) result = (result + dp(n - i)) % MOD;
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    return memo[n] = result;
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  }
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  int peopleAwareOfSecret(int n, int delay, int forget) {
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    this->delay = delay;
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    this->forget = forget;
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    memo.resize(n + 1, -1);
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    return (dp(n) - dp(n - forget) + MOD) % MOD; // Subtract the people who found out by the
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                                                 // `n-forget` day as observed.
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  }
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};