MyQueue.java

package stack;

import java.util.Stack;

/**
 * Created by gouthamvidyapradhan on 29/07/2017.
 * Implement the following operations of a queue using stacks.
 * <p>
 * push(x) -- Push element x to the back of queue.
 * pop() -- Removes the element from in front of queue.
 * peek() -- Get the front element.
 * empty() -- Return whether the queue is empty.
 * <p>
 * Notes:
 * You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
 * Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
 * You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).
 */
public class MyQueue {

    private Stack<Integer> stack;

    public static void main(String[] args) throws Exception {
        MyQueue myQueue = new MyQueue();
        myQueue.push(5);
        myQueue.push(12);
        myQueue.push(7);
        myQueue.push(9);
        System.out.println(myQueue.peek());
        System.out.println(myQueue.pop());
        myQueue.push(56);
        myQueue.push(53);
        System.out.println(myQueue.pop());
    }

    /**
     * Initialize your data structure here.
     */
    public MyQueue() {
        stack = new Stack<>();
    }

    /**
     * Push element x to the back of queue.
     */
    public void push(int x) {
        stack.push(x);
    }

    /**
     * Removes the element from in front of queue and returns that element.
     */
    public int pop() {
        Stack<Integer> auxStack = new Stack<>();
        while (!stack.isEmpty()) {
            auxStack.push(stack.pop());
        }
        int result = auxStack.pop();
        while (!auxStack.isEmpty()) {
            stack.push(auxStack.pop());
        }
        return result;
    }

    /**
     * Get the front element.
     */
    public int peek() {
        Stack<Integer> auxStack = new Stack<>();
        while (!stack.isEmpty()) {
            auxStack.push(stack.pop());
        }
        int result = auxStack.peek();
        while (!auxStack.isEmpty()) {
            stack.push(auxStack.pop());
        }
        return result;
    }

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

}