TrainComposition Solution in Java.

TrainComposition.Java

package javaproblems;

import java.util.Deque;
import java.util.LinkedList;

public class TrainComposition {
    
    private final Deque<Integer> wagons = new LinkedList<>();
    
    public void attachWagonFromLeft(int wagonId) {
       wagons.addFirst(wagonId); 
    }

    public void attachWagonFromRight(int wagonId) {
        wagons.addLast(wagonId); 
    }

    public int detachWagonFromLeft() {
        if(!wagons.isEmpty()){
            return wagons.removeFirst();
        }else{
            return -1;
        }
    }

    public int detachWagonFromRight() {
        if(!wagons.isEmpty()){
           return wagons.removeLast();
        }else{
            return -1;
        }
    }

    public static void main(String[] args) {
        TrainComposition tree = new TrainComposition();
        tree.attachWagonFromLeft(7);
        tree.attachWagonFromLeft(13);
        System.out.println(tree.detachWagonFromRight()); // 7 
        System.out.println(tree.detachWagonFromLeft()); // 13
    }
}