LeetCode 731. My Calendar II

Problem Statement

use std::collections::BTreeMap;

struct MyCalendarTwo {
    events: BTreeMap<i32, i32>
}

/**
 * `&self` means the method takes an immutable reference.
 * If you need a mutable reference, change it to `&mut self` instead.
 */
impl MyCalendarTwo {

    fn new() -> Self {
        MyCalendarTwo {
            events: BTreeMap::new()
        }
    }

    fn book(&mut self, start: i32, end: i32) -> bool {
        self.events.entry(start).or_insert(0);
        self.events.entry(end).or_insert(0);
        *self.events.get_mut(&start).unwrap() += 1;
        *self.events.get_mut(&end).unwrap() -= 1;
        let mut sum = 0;
        for (_k, v) in &self.events {
            sum += *v;
            if sum == 3 {
                *self.events.get_mut(&start).unwrap() -= 1;
                *self.events.get_mut(&end).unwrap() += 1;
                return false;
            }
        }

        return true;
    }
}