731. My Calender II

// 731. My Calendar II
// https://leetcode.com/problems/my-calendar-ii/description/

import java.util.TreeMap;

class MyCalendarTwo {
    private TreeMap<Integer, Integer> delta;

    public MyCalendarTwo() {
        delta = new TreeMap<>(); // Tracks the start and end points of intervals
    }
    
    public boolean book(int start, int end) {
        // Mark the start and end points of the new interval
        delta.put(start, delta.getOrDefault(start, 0) + 1);
        delta.put(end, delta.getOrDefault(end, 0) - 1);
        
        int active = 0; // Tracks the number of active overlaps
        for (int d : delta.values()) {
            active += d; // Update active count based on delta values
            if (active >= 3) { // Triple booking detected
                // Revert the changes if a triple booking is found
                delta.put(start, delta.get(start) - 1);
                delta.put(end, delta.get(end) + 1);
                if (delta.get(start) == 0) delta.remove(start); // Remove if zero count
                if (delta.get(end) == 0) delta.remove(end);     // Remove if zero count
                return false; // Booking is not allowed
            }
        }
        
        return true; // Booking is allowed
    }
}

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