## 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"]
[[], , , [], [], []]
Output
[null, null, null, 1, 1, false]

Explanation
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: 
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is 
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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