232. Implement Queue using Stacks

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Problem

Implement the following operations of a queue using stacks.

push(x) – Push element x to the back of queue. pop() – Removes the element from in front of queue. peek() – Get the front element. empty() – Return whether the queue is empty. Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

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, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack. You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

Code

class MyQueue:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.stack = []

    def push(self, x: int) -> None:
        """
        Push element x to the back of queue.
        """
        temp_stack = []
        while self.stack:
            temp_stack.append(self.stack.pop())

        self.stack.append(x)
        while temp_stack:
            self.stack.append(temp_stack.pop())


    def pop(self) -> int:
        """
        Removes the element from in front of queue and returns that element.
        """
        return self.stack.pop()


    def peek(self) -> int:
        """
        Get the front element.
        """
        return self.stack[-1]


    def empty(self) -> bool:
        """
        Returns whether the queue is empty.
        """
        return not self.stack



# Your MyQueue object will be instantiated and called as such:
# obj = MyQueue()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.peek()
# param_4 = obj.empty()

class MyQueue:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.stack_input = []
        self.stack_output = []


    def push(self, x: int) -> None:
        """
        Push element x to the back of queue.
        """
        self.stack_input.append(x)


    def pop(self) -> int:
        """
        Removes the element from in front of queue and returns that element.
        """
        self.peek()
        return self.stack_output.pop()


    def peek(self) -> int:
        """
        Get the front element.
        """
        if not self.stack_output:
            while self.stack_input:
                self.stack_output.append(self.stack_input.pop())

        return self.stack_output[-1]

    def empty(self) -> bool:
        """
        Returns whether the queue is empty.
        """
        return not self.stack_output and not self.stack_input



# Your MyQueue object will be instantiated and called as such:
# obj = MyQueue()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.peek()
# param_4 = obj.empty()