import edu.princeton.cs.algs4.StdOut;   public class LinkedIntListR {   // helper linked list node class private class Node { private int item; private Node next;   public Node(int item, Node next) { this.item = item; this.next = next; } }   private Node first; // first node of the list   public LinkedIntListR() { first = null; }   public LinkedIntListR(int[] data) { for (int i = data.length - 1; i >= 0; i--) first = new Node(data[i], first); }     public void addFirst(int data) { first = new Node(data, first); }   public void printList() { if (first == null) { StdOut.println("[]"); } else if (first.next == null) { StdOut.println("[" + first.item + "]"); } else { StdOut.print("[" + first.item); for(Node ptr = first.next; ptr != null; ptr = ptr.next) StdOut.print(", " + ptr.item); StdOut.println("]"); } }   @Override public boolean equals(Object obj) { if (this == obj) return true; if (obj == null) return false; if (getClass() != obj.getClass()) return false; LinkedIntListR other = (LinkedIntListR) obj; Node thisPtr = first; Node otherPtr = other.first; while(thisPtr != null && otherPtr != null) { if (thisPtr.item != otherPtr.item) return false; thisPtr = thisPtr.next; otherPtr = otherPtr.next; } if (thisPtr != null || otherPtr != null) return false; return true; }       /********************************** HOMEWORK **********************************/   /** * Computes the sum of all the ints in the list. *  * @return the sum of all the ints in the list */ public int sum() { // TODO throw new RuntimeException("Not implemented"); }     /** * Returns the index of the first occurrence of the specified int in this list, * or -1 if this list does not contain the int. * * @param i int to search for * @return the index of the first occurrence of the specified int in this list, *         or -1 if this list does not contain the int */ public int indexOf(int i) { // TODO throw new RuntimeException("Not implemented"); // HINT:The empty list is not the only base case. //If the first item in the list is the one you are looking for //you should also stop because you have found it. }   /** * Constructs a string representing the list. The ints in the list appear in a * comma separated list contained inside square brackets. *  * @return a string represeting the list. */ public String toString() { // TODO throw new RuntimeException("Not implemented"); // HINT:Have the private method just return comma separted list //and have the public method put the [ and ] around the private //method's answer. //Also, you probably need a base case for a list of size 1 as well //The empty list can be handled in the public method if you want. }     /** * Constructs a new {@code LinkedIntList} consisting of just the even number * from this list appearing in the same order as they occur in this list. *  * @return a new list that looks like this list be with all the odds removed. */ public LinkedIntListR evens() { // TODO throw new RuntimeException("Not implemented"); // HINT:Have the private method return a list via the first node. //Have the public method create an empty list and then set //first in that empty list to the list returned by the helper }   /** * Constructs a new int list where each entry in the new list is the sum of the * corresponding entries in this list and the argument list. *  * @arg otherList the other list to be added to this list * @return a new list where each entry in the new list is the sum of the *         corresponding entries in this list and the argument list * @throws IllegalArgumentException if this list and the other list have have *                                  different lengths */ public LinkedIntListR listAddition(LinkedIntListR otherList) { // TODO throw new RuntimeException("Not implemented"); // HINT:Have the private method return a list via the first node. //Have the public method create an empty list and then set //first in that empty list to the list returned by the helper  }   }