232. Implement Queue using Stacks

Easy

Problem:

Implement a queue using a stack that supports the following operations.

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.

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

Notice: 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.

Solution:

Use two stacks.

class MyQueue:

    def __init__(self):
        self.input = []
        self.output = []

    def push(self, x: int) -> None:
        self.input.append(x)
        
    def pop(self) -> int:
        self.peek()
        return self.output.pop()

    def peek(self) -> int:
        if not self.output:
            while self.input:
                self.output.append(self.input.pop())
        return self.output[-1]

    def empty(self) -> bool:
        return self.input = [] and self.output == []