My Calendar III LeetCode solution in C++

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class MyCalendarThree {
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 public:
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  map<int, int> mp;
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  MyCalendarThree() {}
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  int book(int start, int end) {
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    mp[start]++;
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    mp[end]--;
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    int sum = 0;
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    int ans = 0;
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    for (auto it = mp.begin(); it != mp.end(); it++) {
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      sum += it->second;
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      ans = max(ans, sum);
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    }
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    return ans;
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  }
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};