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]

Explanation
MyQueue 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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