LeetCode #225 Implement Stack using Queues

Easy

Problem

Implement the following operations of a stack using queues.

• push(x) — Push element x onto stack.
• pop() — Removes the element on top of the stack.
• top() — Get the top element.
• empty() — Return whether the stack is empty.

Example:

MyStack stack = new MyStack();
stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

Notes:

• You must use only standard operations of a queue — which means only push to back, peek/pop from front, size, and is emptyoperations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

Solution

We will bridge pop/top/empty manipulations of this stack to pop/peek/isEmpty manipulations of queue. However, for push operation, we have to reverse rest elements after one new element entering.

Complexity

It’s trivial that pop/top/empty manipulations only take O(1) time, but for push operation, it takes O(n) time if n denotes to all elements which have to be handled.