## 232. Implement Queue using Stacks

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (`push`

, `peek`

, `pop`

, and `empty`

).

Implement the `MyQueue`

class:

`void push(int x)`

Pushes element x to the back of the queue.`int pop()`

Removes the element from the front of the queue and returns it.`int peek()`

Returns the element at the front of the queue.`boolean empty()`

Returns`true`

if the queue is empty,`false`

otherwise.

**Notes:**

- You must use
**only**standard operations of a stack, which means only`push to top`

,`peek/pop from top`

,`size`

, and`is empty`

operations are valid. - Depending on your language, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack's standard operations.

**Follow-up:** Can you implement the queue such that each operation is **amortized** `O(1)`

time complexity? In other words, performing `n`

operations will take overall `O(n)`

time even if one of those operations may take longer.

**Example 1:**

Input["MyQueue", "push", "push", "peek", "pop", "empty"] [[], [1], [2], [], [], []]Output[null, null, null, 1, 1, false]ExplanationMyQueue myQueue = new MyQueue(); myQueue.push(1); // queue is: [1] myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue) myQueue.peek(); // return 1 myQueue.pop(); // return 1, queue is [2] myQueue.empty(); // return false

**Constraints:**

`1 <= x <= 9`

- At most
`100`

calls will be made to`push`

,`pop`

,`peek`

, and`empty`

. - All the calls to
`pop`

and`peek`

are valid.

## Rust Solution

```
#[derive(Default)]
struct MyQueue {
stack: Vec<i32>,
temp: Vec<i32>,
}
impl MyQueue {
fn new() -> Self {
MyQueue {
stack: vec![],
temp: vec![],
}
}
fn push(&mut self, x: i32) {
self.stack.push(x);
}
fn pop(&mut self) -> i32 {
while let Some(x) = self.stack.pop() {
self.temp.push(x);
}
let res = self.temp.pop().unwrap();
while let Some(x) = self.temp.pop() {
self.stack.push(x)
}
res
}
fn peek(&mut self) -> i32 {
while let Some(x) = self.stack.pop() {
self.temp.push(x);
}
let res = self.temp.pop().unwrap();
self.stack.push(res);
while let Some(x) = self.temp.pop() {
self.stack.push(x)
}
res
}
fn empty(&self) -> bool {
self.stack.is_empty()
}
}
#[test]
fn test() {
let mut queue = MyQueue::new();
queue.push(1);
queue.push(2);
assert_eq!(queue.peek(), 1);
assert_eq!(queue.pop(), 1);
assert_eq!(queue.empty(), false);
}
```

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