LeetCode 225. Implement Stack using Queues

Description

https://leetcode.com/problems/implement-queue-using-stacks/

Implement a last in first out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal queue (pushtoppop, 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 backpeek/pop from frontsize, 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 pushpoptop, and empty.
  • All the calls to pop and top are valid.

Follow-up: Can you implement the stack such that each operation is amortizedO(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.

Explanation

Using two queues, one queue is for getting items, the other queue is only for internal conversion to get the last item.

Python Solution

import queue
class MyStack:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.queue1 = queue.Queue()
        self.queue2 = queue.Queue()
        

    def push(self, x: int) -> None:
        """
        Push element x onto stack.
        """
        self.queue1.put(x)

    def pop(self) -> int:
        """
        Removes the element on top of the stack and returns that element.
        """
        while self.queue1.qsize() > 1:
            self.queue2.put(self.queue1.get())

        
        top = self.queue1.get()
        
        self.queue1, self.queue2 = self.queue2, self.queue1
        
        return top
        

    def top(self) -> int:
        """
        Get the top element.
        """
        while self.queue1.qsize() > 1:
            self.queue2.put(self.queue1.get())

        top = self.queue1.get()
        self.queue2.put(top)
        
        self.queue1, self.queue2 = self.queue2, self.queue1
        
        return top
        

    def empty(self) -> bool:
        """
        Returns whether the stack is empty.
        """
        return not self.queue1.qsize()


# Your MyStack object will be instantiated and called as such:
# obj = MyStack()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.top()
# param_4 = obj.empty()
  • Time complexity: O(N).
  • Space complexity: O(1).

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