HackerRank - Cracking the Code Interview - Queues: A Tale of Two Stacks (https://www.hackerrank.com/challenges/ctci-queue-using-two-stacks)

Solution.java

import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

public class Solution {
    
    private static class MyQueue<T> {
        
        private Stack<T> in = new Stack<T>();
        private Stack<T> out = new Stack<T>();
        
        public void enqueue(T item) {
            in.push(item);
        }
        
        public T dequeue() {
            if (out.isEmpty()) {
                while (!in.empty()) {
                    out.push(in.pop());
                }
            }
            
            return out.pop();
        }
        
        public T peek() {
            if (out.isEmpty()) {
                while (!in.empty()) {
                    out.push(in.pop());
                }
            }
            
            return out.peek();
        }
        
    }
    
    public static void main(String[] args) {
        MyQueue<Integer> queue = new MyQueue<Integer>();      

        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();

        for (int i = 0; i < n; i++) {
            int operation = scan.nextInt();
            if (operation == 1) { // enqueue
              queue.enqueue(scan.nextInt());
            } else if (operation == 2) { // dequeue
              queue.dequeue();
            } else if (operation == 3) { // print/peek
              System.out.println(queue.peek());
            }
        }
        scan.close();
    }
}