225. Implement Stack using Queues

Implement a last in first out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal queue (push, top, pop, and empty).

Implement the MyStack class:

  • void push(int x) Pushes element x to the top of the stack.
  • int pop() Removes the element on the top of the stack and returns it.
  • int top() Returns the element on the top of the stack.
  • boolean empty() Returns true if the stack is empty, false otherwise.

Notes:

  • You must use only standard operations of a queue, which means only push to back, peek/pop from front, size, and is empty operations are valid.
  • Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue), as long as you use only a queue's standard operations.

 

Example 1:

Input
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]

Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False

 

Constraints:

  • 1 <= x <= 9
  • At most 100 calls will be made to push, pop, top, and empty.
  • All the calls to pop and top are valid.

 

Follow-up: Can you implement the stack 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. You can use more than two queues.

Rust Solution

use std::collections::VecDeque;

#[derive(Default)]
struct MyStack {
    queue: VecDeque<i32>,
}

impl MyStack {
    fn new() -> Self {
        MyStack {
            queue: VecDeque::new(),
        }
    }

    fn push(&mut self, x: i32) {
        let mut n = self.queue.len();
        self.queue.push_back(x);
        while n > 0 {
            let front = self.queue.pop_front().unwrap();
            self.queue.push_back(front);
            n -= 1;
        }
    }

    fn pop(&mut self) -> i32 {
        self.queue.pop_front().unwrap()
    }

    fn top(&self) -> i32 {
        *self.queue.front().unwrap()
    }

    fn empty(&self) -> bool {
        self.queue.is_empty()
    }
}

#[test]
fn test() {
    let mut stack = MyStack::new();
    stack.push(1);
    stack.push(2);
    assert_eq!(stack.top(), 2);
    assert_eq!(stack.pop(), 2);
    assert_eq!(stack.empty(), false);
}

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