Description

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).

Solutions

1. Two Stacks

# Time: O(n)
# Space: O(n)
class MyQueue:

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

    def pour(self, push_stack, helper):
        if not any(push_stack):
            return
        while push_stack:
            helper.append(push_stack.pop())

    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 not any(self.push_stack) and not self.helper:
            return None

        if not any(self.helper):
            self.pour(self.push_stack, self.helper)

        return self.helper.pop()

    def peek(self) -> int:
        """
        Get the front element.
        """
        if not any(self.push_stack) and not self.helper:
            return None
        
        if not any(self.helper):
            self.pour(self.push_stack, self.helper)
        
        return self.helper[-1]

    def empty(self) -> bool:
        """
        Returns whether the queue is empty.
        """
        if not any(self.push_stack) and not self.helper:
            return True
        else:
            return False

# 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()
# 17/17 cases passed (20 ms)
# Your runtime beats 98.55 % of python3 submissions
# Your memory usage beats 100 % of python3 submissions (12.8 MB)

References

  1. 232. Implement Queue using Stacks