My Calendar III LeetCode solution in Java

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class MyCalendarThree {
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  TreeMap<Integer, Integer> map;
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  public MyCalendarThree() {
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    map = new TreeMap<>();
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  }
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  public int book(int start, int end) {
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    if (map.isEmpty()) {
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      map.put(start, 1);
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      map.put(end, -1);
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      return 1;
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    }
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    // upvote if you like the solution
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    map.put(start, map.getOrDefault(start, 0) + 1);
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    map.put(end, map.getOrDefault(end, 0) - 1);
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    int res = 0;
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    int sum = 0;
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    for (Map.Entry<Integer, Integer> e : map.entrySet()) {
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      sum += e.getValue();
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      res = Math.max(res, sum);
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    }
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    return res;
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  }
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}