LeetCode 225. Implement Stack using Queues (Java)

Source: https://leetcode.com/problems/implement-stack-using-queues/

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 backpeek/pop from frontsize, and is empty operations 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).

Other parts are simple, we can just call methods of the queue. The hard part is the push method, our method is to create a temporary queue to add the new value, then add all the elements of the old queue, then use the new queue as internal storage.

Github: https://github.com/tinyfool/leetcode/tree/master/src/p0225

Other problems related to stack and queue, see also Stack and Queue Data Structure (Java code and Leetcode problems)

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