Is Implement Stack using Queues easy or hard?

LeetCode classifies this problem as Easy because it mainly tests understanding of stack and queue behavior rather than complex algorithms. The key idea is reversing element order using queue operations to maintain LIFO semantics.

Implement Stack using Queues Python/Java solution

In Python or Java, the queue can be implemented using collections like deque in Python or LinkedList/ArrayDeque in Java. After each push, rotate the queue elements so the most recently added value moves to the front, allowing pop and top to run in constant time.

How to solve Implement Stack using Queues in O(n)?

Insert the new element into a queue and then rotate the queue so that the newly added element moves to the front. This requires moving the previous elements to the back one by one. After rotation, the queue front always represents the stack top, enabling constant-time pop operations.

What is the best approach for Implement Stack using Queues?

The single queue rotation approach is generally considered the best solution. After pushing an element into the queue, rotate the queue by moving the previous elements behind it so the newest element becomes the front. This keeps pop() and top() operations O(1) while push() takes O(n).

Is Implement Stack using Queues asked at Google/Amazon/Meta?

Stack and queue design problems frequently appear in interviews at companies like Amazon, Google, and Meta. This specific question tests understanding of data structure behavior and the ability to simulate one structure using another with constrained operations.

What data structure is used in Implement Stack using Queues?

The implementation uses one or two queues to simulate stack behavior. Queue operations such as enqueue, dequeue, and front access are used to reorder elements so the newest element behaves like the top of a stack.

What is the time complexity of Implement Stack using Queues?

Most implementations achieve push in O(n) time and pop in O(1) time. The push operation requires rearranging or rotating elements to maintain stack order. Space complexity is O(n) because all elements are stored inside one or two queues.

Description Solution Video 7:29

225. Implement Stack using Queues

Easy70.1% acceptance9 companies

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: