Name: 232. Implement Queue using Stacks
Rating: 2.4 (6344 reviews)
Author: seokjeon

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.

Idea #1 (Time: ?, Memory: ?)

push: stack push
pop: iterate until next node of stack is None and return node.data with prev.next = None
peek: same as pop but not prev.next = None
empty: stack.isempty()

Test Cases

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
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false

Code



class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

class Stack:
    def __init__(self):
        self.top = None

    def push(self, data):
        if self.top is None:
            self.top = Node(data)
        else:
            node = Node(data)
            node.next = self.top
            self.top = node

    def pop(self):
        if self.top is None:
            return None
        node = self.top
        self.top = self.top.next
        return node.data

    def peek(self):
        if self.top is None:
            return None
        return self.top.data

    def is_empty(self):
        return self.top is None

class MyQueue:
    def __init__(self):
        self.mystack = Stack()

    def push(self, x: int) -> None:
        self.mystack.push(x)

    def pop(self) -> int:
        if self.mystack.top.next is None: return self.mystack.pop()
        node = self.mystack.top
        prev = None
        while node.next:
            prev = node
            node = node.next
        res = node.data
        prev.next = None
        return res


    def peek(self) -> int:
        node = self.mystack.top
        prev = None
        while node:
            prev = node
            node = node.next
        return prev.data

    def empty(self) -> bool:
        return self.mystack.is_empty()