Leetcode 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.

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

• To implement queue by using stack, the push and empty will be exactly same as stack. For pop and top operations, we need to reorder the number in stack. However, since the operations are dynamic changing, to save the cost, the best way is to create extra space called pop_stack to append number that pop out from original stack, this will reverse order will minimum cost.

class MyQueue:

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

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

    def pop(self) -> int:
        """
        Removes the element from in front of queue and returns that element.
        """
        if self.empty(): return
        if len(self.pop_stack):
            return self.pop_stack.pop()
        else:
            while len(self.push_stack):
                self.pop_stack.append(self.push_stack.pop())
        return self.pop_stack.pop()

    def peek(self) -> int:
        """
        Get the front element.
        """
        if self.empty(): return
        if len(self.pop_stack):
            return self.pop_stack[-1]
        else:
            while len(self.push_stack):
                self.pop_stack.append(self.push_stack.pop())
        return self.pop_stack[-1]

    def empty(self) -> bool:
        """
        Returns whether the queue is empty.
        """
        return len(self.push_stack)==False and len(self.pop_stack) == False