## 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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