ImplementStackUsingQueues.java

import java.util.ArrayDeque;
import java.util.Deque;
import java.util.LinkedList;
import java.util.Queue;

/**
 * Implement the following operations of a stack using queues.
 * <p>
 * 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.
 * Notes:
 * You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, 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).
 * Update (2015-06-11):
 * The class name of the Java function had been updated to MyStack instead of Stack.
 * <p>
 * Tags: Stack, Design
 * Similar Problems: (E) Implement Queue using Stacks
 */
public class ImplementStackUsingQueues {

    /**
     * Use queue
     * It would too easy to use Deque
     */
    class MyStack {

        Queue<Integer> queue = new LinkedList<>();

        // Push element x onto stack.
        public void push(int x) {
            queue.offer(x);
            for (int i = 0; i < queue.size() - 1; i++) {
                queue.offer(queue.poll());
            }
        }

        // Removes the element on top of the stack.
        public void pop() {
            queue.poll();
        }

        // Get the top element.
        public int top() {
            return queue.peek();
        }

        // Return whether the stack is empty.
        public boolean empty() {
            return queue.isEmpty();
        }
    }
}