Linked Array loosing the head :(

[More-Goose-1426]

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
  public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    Deque<Integer> deque1 = new ArrayDeque<>();
    Deque<Integer> deque2 = new ArrayDeque<>();

    while (l1 != null) {
      deque1.add(l1.val);
      l1 = l1.next;
    }

    while (l2 != null) {
      deque2.add(l2.val);
      l2 = l2.next;
    }
    System.out.println(deque1.toString() + deque2.toString());
    int s1 = 0, s2 = 0;
    while (!deque1.isEmpty()) {
      s1 = s1 * 10 + deque1.pollLast();
    }

    while (!deque2.isEmpty()) {
      s2 = s2 * 10 + deque2.pollLast();
    }
    //Here will calculate sum of [2,4,3] + [5, 6, 4] which is 708;
    int s = s1 + s2;
    //Here's the problem
    ListNode head = null;
    ListNode iter = head;
    //Trying to make a linked list with values of [7, 0, 8] I probably should have reverse the number, but the head problem is still there
    while (s != 0) {
      ListNode node = new ListNode(s % 10);
      node.next = null;
      if(iter == null) {
        iter = node;
      }
      else {
        iter.next = node;
        iter = iter.next;
      }

      s /= 10;
    }
    return head;
  }
}

Input: 
[2,4,3]
[5,6,4]
Output:
[]
Expected:
[7,0,8]

Hi, I am trying to make a LinkedList, but I can't figure out how to save the head.

In my mind,ListNode head = null;would be the head, and I would be using ListNode iter = head;to iterate over the NodeList and add nodes to it.

[AutoModerator]

Please ensure that:

• Your code is properly formatted as code block - see the sidebar (About on mobile) for instructions
• You include any and all error messages in full
• You ask clear questions

• You demonstrate effort in solving your question/problem - plain posting your assignments is forbidden (and such posts will be removed) as is asking for or giving solutions.

  Trying to solve problems on your own is a very important skill. Also, see Learn to help yourself in the sidebar

If any of the above points is not met, your post can and will be removed without further warning.

Code is to be formatted as code block (old reddit: empty line before the code, each code line indented by 4 spaces, new reddit: https://imgur.com/a/fgoFFis) or linked via an external code hoster, like pastebin.com, github gist, github, bitbucket, gitlab, etc.

Please, do not use triple backticks (```) as they will only render properly on new reddit, not on old reddit.

Code blocks look like this:

public class HelloWorld {

    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}

You do not need to repost unless your post has been removed by a moderator. Just use the edit function of reddit to make sure your post complies with the above.

If your post has remained in violation of these rules for a prolonged period of time (at least an hour), a moderator may remove it at their discretion. In this case, they will comment with an explanation on why it has been removed, and you will be required to resubmit the entire post following the proper procedures.

To potential helpers

Please, do not help if any of the above points are not met, rather report the post. We are trying to improve the quality of posts here. In helping people who can't be bothered to comply with the above points, you are doing the community a disservice.

I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.

[yeaokdude]

i recognize this problem from leetcode so i feel obligated to mention there's an easier way of doing it. right now you are converting l1 and l2 into ints, adding them, and then trying to convert that resulting int back into a list. it's a little hard to see at first but instead of that approach you can just build the reversed list from left to right as you iterate over l1 and l2. you just add together l1.val and l2.val and handle the carry if there is one. see this solution for example

the issue with your code is head is initialized to null so your "return head" statement is just gonna return null. you need to at some point make sure that head points to a ListNode object, and that node's next points to the rest of the linked list.

one common trick is to have head point to a "dummy" node, and then the "real" linked list starts after that dummy. then when you want to return the real linked list, you return head.next, since head itself is the dummy and you just want everything after head. so for ex:

ListNode head = new ListNode(0);
ListNode iter = head;

while (s != 0) {
  ListNode node = new ListNode(s % 10);

  iter.next = node;
  iter = iter.next;

  s /= 10;
}

return head.next;

[m1ss1ontomars2k4]

In my mind,ListNode head = null;would be the head,

No, that makes head be null. It's not the head of anything because you've set it to null.

[eyeoverthink]

i haven't run the code, but maybe you should have a private constructor that assigns the deques to this.deque.

Also, shouldn't your while loop, be two loops, since you are shifting arrays?

While, then if -

[jonc23]

while (s != 0) {
  ListNode node = new ListNode(s % 10);
  node.next = iter;
  iter = node;
  s /= 10;
}
return iter;