## 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:**

Input:["FreqStack","push","push","push","push","push","push","pop","pop","pop","pop"], [[],[5],[7],[5],[7],[4],[5],[],[],[],[]]Output:[null,null,null,null,null,null,null,5,7,5,4]Explanation: 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].

**Note:**

- 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;
#[derive(Default)]
struct FreqStack {
freq: HashMap<i32, usize>,
stacks: HashMap<usize, Vec<i32>>,
max_freq: usize,
}
impl FreqStack {
fn new() -> Self {
FreqStack::default()
}
fn push(&mut self, x: i32) {
let n = self.freq.entry(x).or_default();
*n += 1;
self.stacks.entry(*n).or_default().push(x);
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;
}
x
}
}
#[test]
fn test() {
let mut obj = FreqStack::new();
obj.push(5);
obj.push(7);
obj.push(5);
obj.push(7);
obj.push(4);
obj.push(5);
assert_eq!(obj.pop(), 5);
assert_eq!(obj.pop(), 7);
assert_eq!(obj.pop(), 5);
assert_eq!(obj.pop(), 4);
}
```

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