题目描述：

LeetCode 732. My Calendar III

Implement a MyCalendarThree class to store your events. A new event can always be added.

Your class will have one method, book(int start, int end). Formally, this represents a booking on the half open interval [start, end), the range of real numbers x such that start <= x < end.

A K-booking happens when K events have some non-empty intersection (ie., there is some time that is common to all K events.)

For each call to the method MyCalendar.book, return an integer K representing the largest integer such that there exists a K-booking in the calendar.

Your class will be called like this: MyCalendarThree cal = new MyCalendarThree(); MyCalendarThree.book(start, end)

Example 1:

MyCalendarThree();
MyCalendarThree.book(10, 20); // returns 1
MyCalendarThree.book(50, 60); // returns 1
MyCalendarThree.book(10, 40); // returns 2
MyCalendarThree.book(5, 15); // returns 3
MyCalendarThree.book(5, 10); // returns 3
MyCalendarThree.book(25, 55); // returns 3
Explanation: 
The first two events can be booked and are disjoint, so the maximum K-booking is a 1-booking.
The third event [10, 40) intersects the first event, and the maximum K-booking is a 2-booking.
The remaining events cause the maximum K-booking to be only a 3-booking.
Note that the last event locally causes a 2-booking, but the answer is still 3 because
eg. [10, 20), [10, 40), and [5, 15) are still triple booked.

Note:

• The number of calls to MyCalendarThree.book per test case will be at most 1000.
• In calls to MyCalendarThree.book(start, end), start and end are integers in the range [0, 10^9].

题目大意：

记录一组活动，求当前所有活动的最大重叠次数。

解题思路：

TreeMap（红黑树）

利用TreeMap记录时间端点的次数，起点计数+1，终点计数-1。

每增加一个活动，遍历TreeMap并累加，累计值的最大值即为当前的最大重叠活动次数。

p.s. TreeMap的get操作时间开销为O( log n )，换用keySet+get方式遍历TreeMap会导致TLE。

Java代码：

class MyCalendarThree {

    private Map<Integer, Integer> dmap;
    public MyCalendarThree() {
        dmap = new TreeMap<>();
    }
    
    public int book(int start, int end) {
        int cnt = 0, ans = 0;
        dmap.put(start, dmap.getOrDefault(start, 0) + 1);
        dmap.put(end, dmap.getOrDefault(end, 0) - 1);
        for (int key : dmap.keySet()) {
            cnt += dmap.get(key);
            ans = Math.max(ans, cnt);
        }
        return ans;
    }
}

/**
 * Your MyCalendarThree object will be instantiated and called as such:
 * MyCalendarThree obj = new MyCalendarThree();
 * int param_1 = obj.book(start,end);
 */

 

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