Number of Days Between Two Dates LeetCode solution in C++

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class Solution {
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 public:
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  int days[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
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  bool isLeap(int y) {
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    return (y % 4 == 0 && (y % 100 != 0 || y % 400 == 0));
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  }
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  int calc(string s) {
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    int y = stoi(s.substr(0, 4));
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    int m = stoi(s.substr(5, 2));
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    int d = stoi(s.substr(8));
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    for (int i = 1971; i < y; i++) d += isLeap(i) ? 366 : 365;
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    d += accumulate(begin(days), begin(days) + m - 1, 0);
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    d += (m > 2 && isLeap(y)) ? 1 : 0;
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    return d;
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  }
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  int daysBetweenDates(string date1, string date2) {
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    int ans = abs(calc(date2) - calc(date1));
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    return ans;
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  }
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};