895. Maximum Frequency Stack

Implement FreqStack, a class which simulates the operation of a stack-like data structure.

FreqStack has two functions:

  • push(int x), which pushes an integer x onto the stack.
  • pop(), which removes and returns the most frequent element in the stack.
    • If there is a tie for most frequent element, the element closest to the top of the stack is removed and returned.


Example 1:

Output: [null,null,null,null,null,null,null,5,7,5,4]
After making six .push operations, the stack is [5,7,5,7,4,5] from bottom to top.  Then:

pop() -> returns 5, as 5 is the most frequent.
The stack becomes [5,7,5,7,4].

pop() -> returns 7, as 5 and 7 is the most frequent, but 7 is closest to the top.
The stack becomes [5,7,5,4].

pop() -> returns 5.
The stack becomes [5,7,4].

pop() -> returns 4.
The stack becomes [5,7].



  • Calls to FreqStack.push(int x) will be such that 0 <= x <= 10^9.
  • It is guaranteed that FreqStack.pop() won't be called if the stack has zero elements.
  • The total number of FreqStack.push calls will not exceed 10000 in a single test case.
  • The total number of FreqStack.pop calls will not exceed 10000 in a single test case.
  • The total number of FreqStack.push and FreqStack.pop calls will not exceed 150000 across all test cases.


Rust Solution

use std::collections::HashMap;

struct FreqStack {
    freq: HashMap<i32, usize>,
    stacks: HashMap<usize, Vec<i32>>,
    max_freq: usize,

impl FreqStack {
    fn new() -> Self {

    fn push(&mut self, x: i32) {
        let n = self.freq.entry(x).or_default();
        *n += 1;
        self.max_freq = self.max_freq.max(*n);

    fn pop(&mut self) -> i32 {
        let max_stack: &mut Vec<i32> = self.stacks.get_mut(&self.max_freq).unwrap();
        let x = max_stack.pop().unwrap();
        *self.freq.entry(x).or_default() -= 1;
        if max_stack.is_empty() {
            self.max_freq -= 1;

fn test() {
    let mut obj = FreqStack::new();
    assert_eq!(obj.pop(), 5);
    assert_eq!(obj.pop(), 7);
    assert_eq!(obj.pop(), 5);
    assert_eq!(obj.pop(), 4);

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