Implement Queue using Stacks — LeetCode 232 Python Solution

The problem

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).

Example

Input

["MyQueue", "push", "push", "peek", "pop", "empty"]

Output

[null, null, null, 1, 1, false]

Explanation

MyQueue myQueue = new MyQueue();

Python solution

class MyQueue:
    def __init__(self):
        self.stk1 = []
        self.stk2 = []

    def push(self, x: int) -> None:
        self.stk1.append(x)

    def pop(self) -> int:
        self.move()
        return self.stk2.pop()

    def peek(self) -> int:
        self.move()
        return self.stk2[-1]

    def empty(self) -> bool:
        return not self.stk1 and not self.stk2

    def move(self):
        if not self.stk2:
            while self.stk1:
                self.stk2.append(self.stk1.pop())


# 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()

Complexity

Pattern: Stack

When the most recent unresolved thing is the one that matters, use a stack. LeetCode 232. Implement Queue using Stacks is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Stack and Queue.

The stack guide has the Python template for the pattern and the 194 LeetCode problems that use it.

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