How can I insert an Item to the front of the List?-CodePudding

I have a method called "public void insertAt(int index, int item)". This method is meant to "Insert an item at position index, where the index is passed to the method" I have my code for this method down below. When I insert an Item at an Index it works unless it's the first item in the list. When I try to insert an item at the beginning of the list nothing happens. For example, If I had a list: "[9, 8, 15, 7, 5, 15, 19, 6, 19, 2]" and I want to insert the number "90" in the first node it should look like [90, 9, 8, 15, 7, 5, 15, 19, 6, 19, 2] but instead I get "[9, 8, 15, 7, 5, 15, 19, 6, 19, 2]". How can I fix this in my code so if I was to insert an item at the head it would move the Item I want inserted to be placed at the head and every other item to be shifted down the list?

import java.util.Random;

public class LinkedListOfInts {
    Node head;
    Node tail;

    private class Node {
        int value;
        Node nextNode;

        public Node(int value, Node nextNode) {
            this.value = value;
            this.nextNode = nextNode;
        }

    }

    public LinkedListOfInts(int N, int low, int high) {
        Random random = new Random();
        for (int i = 0; i < N; i  )
            this.addToFront(random.nextInt(high - low)   low);
    }

    public void addToFront(int x) {
        head = new Node(x, head);
    }

    public void insertAt(int index, int item) {
        Node temp = head;
        Node prev = null;
        int i = 0;
        for (Node ptr = head; ptr != null; ptr = ptr.nextNode) {
            if (index == i) {
                Node newItem = new Node(item, null);
                if (prev != null) {
                    prev.nextNode = newItem;
                }
                newItem.nextNode = temp;
            }
            if (temp.nextNode != null) {
                prev = temp;
                temp = temp.nextNode;
                i  ;
            }
        }
    }

    public String toString() {
        String result = "";
        for (Node ptr = head; ptr != null; ptr = ptr.nextNode) {
            if (!result.isEmpty()) {
                result  = ", ";
            }
            result  = ptr.value;
        }
        return "["   result   "]";
    }

    public static void main(String[] args) {
        LinkedListOfInts list = new LinkedListOfInts(10, 1, 20);
        System.out.println(list.toString());
        list.insertAt(0, 27);
        System.out.println(list.toString());
    }
}

CodePudding user response：

It's because you never connect it to the rest of the list when its the head. All you need to do is add a one-line else statement for when the previous Node is null, like so

       for (Node ptr = head; ptr != null; ptr = ptr.nextNode) {
               if (index == i) {
                    Node newItem = new Node(item, null);
                    if (prev != null) {
                         prev.nextNode = newItem;
                    } else {
                         head = newItem;
                    }
                    newItem.nextNode = temp;
                    break;

(I threw in the break for performance, no need to iterate through the rest of the values once you've inserted the new node)

CodePudding user response：

you can add condition if index == 0 then just add a new node in from of the list.

here is your function

public void insertAt(int index, int item) {
        if (index == 0) {
            Node node = new Node(item, null);
            node.nextNode = head;
            head = node;
        }
        else {
            Node temp = head;
            Node prev = null;
            int i = 0;
            for (Node ptr = head; ptr != null; ptr = ptr.nextNode) {
                if (index == i) {
                    Node newItem = new Node(item, null);
                    if (prev != null) {
                        prev.nextNode = newItem;
                    }
                    newItem.nextNode = temp;
                }
                if (temp.nextNode != null) {
                    prev = temp;
                    temp = temp.nextNode;
                    i  ;
                }
            }
        }
    }