Implement Stack using Queues

Implement Stack using Queues is a Easy Data Structures and Algorithms interview problem you can solve, run and submit on Thita.ai. It belongs to the Design Patterns pattern, in the Design (General/Specific) subpattern.

Problem statement

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).

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 that only push to back, peek/pop from front, size 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 push, pop, top, and empty.

- All the calls to pop and top are valid.

Follow-up: Can you implement the stack using only one queue?

Topics and companies

Topics: Stack, Design, Queue.

Reported in interviews at Amazon, Paypal.

How to practise Implement Stack using Queues on Thita.ai

Starter code is provided in C, C#, C++, Go, Java, JavaScript and Python. Your solution runs against 5 test cases for this problem, with the AI coach available for a hint when you are stuck rather than a finished answer. The reference solution runs in Push: O(n), Pop: O(1), Top: O(1), Empty: O(1). Push is O(n) because all elements are moved from q1 to q2 on each push. Pop, top, and empty are O(1) as they operate on the front of q1. time and O(n), where n is the number of elements in the stack. Both queues together never hold more than n elements in total. space.

Open Implement Stack using Queues in the code editor, or read the Implement Stack using Queues editorial for a worked solution with its approach and complexity analysis.

Where Implement Stack using Queues sits in the DSA pattern sheet

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