Implement Stack using Queues — LeetCode 225 Python Solution

The problem

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Example

Input

["MyStack", "push", "push", "top", "pop", "empty"]

Output

[null, null, null, 2, 2, false]

Explanation

MyStack myStack = new MyStack();

Python solution

class MyStack:
    def __init__(self):
        self.q1 = deque()
        self.q2 = deque()

    def push(self, x: int) -> None:
        self.q2.append(x)
        while self.q1:
            self.q2.append(self.q1.popleft())
        self.q1, self.q2 = self.q2, self.q1

    def pop(self) -> int:
        return self.q1.popleft()

    def top(self) -> int:
        return self.q1[0]

    def empty(self) -> bool:
        return len(self.q1) == 0


# 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()

Complexity

Pattern: Stack

When the most recent unresolved thing is the one that matters, use a stack. LeetCode 225. Implement Stack using Queues 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.

Related problems