Clumsy Factorial LeetCode solution in C++

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class Solution {
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 public:
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  int calc(int num) {
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    int a = num, b = num - 1, c = num - 2;
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    return a * b / c;
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  }
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  int clumsy(int n) {
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    if (n == 1) return 1;
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    if (n == 2) return 2;
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    if (n == 3) return 6;
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    int res = calc(n), i;
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    res += n - 3;
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    for (i = n - 4; i >= 4; i -= 4) {
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      int val = calc(i);
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      res -= val;
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      res += i - 3;
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    }
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    if (i == 1)
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      return res - 1;
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    else if (i == 2)
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      return res - 2;
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    else if (i == 3)
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      return res - 6;
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    return res;
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  }
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};