LeetCode 895. Maximum Frequency Stack

Problem Statement

use std::collections::HashMap;
use std::cmp;

struct FreqStack {
    mx_freq: i32,
    freq: HashMap<i32, i32>,
    m: HashMap<i32, Vec<i32>>
}

impl FreqStack {

    fn new() -> Self {
        FreqStack {
            mx_freq: 0,
            freq: HashMap::new(),
            m: HashMap::new()
        }
    }

    fn push(&mut self, x: i32) {
        self.freq.entry(x).or_insert(0);
        *self.freq.get_mut(&x).unwrap() += 1;
        let val = *self.freq.get(&x).unwrap();
        self.mx_freq = cmp::max(self.mx_freq, val);
        self.m.entry(val).or_insert(vec![]);
        self.m.get_mut(&val).unwrap().push(x);
    }

    fn pop(&mut self) -> i32 {        
        let x = self.m.get(&self.mx_freq).unwrap().last().unwrap().clone();
        self.m.get_mut(&self.mx_freq).unwrap().pop();
        let val = self.freq.get(&x).unwrap();
        if !self.m.contains_key(val) || self.m.get(val).unwrap().is_empty() {
            self.mx_freq -= 1;
        }
        *self.freq.get_mut(&x).unwrap() -= 1;
        return x;
    }
}