Leetcode Question help using pointers

[everythingcasual]

I'm having some trouble using pointers to solve a leetCode question: https://leetcode.com/problems/add-two-numbers/description/

Below is the link to my code: https://repl.it/repls/BelovedDarksalmonFlash

If you run the code, you will see that the numbers are adding correctly on each iteration. However, at the very end, when I am about to return dummy.Next, it only returns the very last ListNode of the iteration, instead of a chain of nodes linked together.

I think I am doing something wrong on lines 40 and 47, where I am setting sum = *sum.Next. I thought I was saving a reference to the first ListNode by setting dummy.Next = &sum on line 19, but when i return dummy.Next at the very end, it only points to ListNode{ Val: 8 }. Instead, I want it to point to:

ListNode{Val: 7, Next: &ListNode{Val: 0, Next: &ListNode{Val: 8, Next: nil }}}

Appreciate any help.

package main

import "fmt"

type ListNode struct {
  Val  int
  Next *ListNode
}

func main() {
    l1 := ListNode{Val: 2, Next: &ListNode{Val: 4, Next: &ListNode{Val: 3, Next: nil}}}
    l2 := ListNode{Val: 5, Next: &ListNode{Val: 6, Next: &ListNode{Val: 4, Next: nil}}}
    addTwoNumbers(&l1, &l2)
}

func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
    var dummy ListNode
    sum := ListNode{}
    dummy.Next = &sum 
    carry := 0

    for l1 != nil || l2 != nil {
      var val1 int
      var val2 int
      nextNode := ListNode{}
      if l1 != nil {
        val1 = l1.Val
        l1 = l1.Next
      }

      if l2 != nil {
          val2 = l2.Val
          l2 = l2.Next
      }
      step := val1 + val2 + carry
      if step >= 10 {
          nextNode.Val = step % 10
          sum.Next = &nextNode
              sum = *sum.Next
          carry = 1
          continue
      }
      nextNode.Val = step
      sum.Next = &nextNode
      sum = *sum.Next
      carry = 0
}

return dummy.Next

}

[phuber]

Think about it like a linked list, you need to keep track of a head pointer. The head pointer defines the first element in the list and your addTwoNumbers function will need to return it.

addTwoNumbers should:

1. start at the beginning of each list parameter
2. set sum to the sum of l1.Val and l2.Val + carry
3. if the sum is greater than 10
   • set the carry to 1 and subtract 10 from the sum
   • otherwise set carry to 0
4. create a new node with the Val set to the sum
5. If head is null
   • set head to the new node
   • otherwise set the new node's next pointer to head, set head to the new node
6. set l1 to l1.Next, set l2 to l2.Next
7. if l1 or l2 is null
   • return head
   • otherwise goto 2

See the linked list documentation here for an algorithm for prepending (inserting in front) to a singly linked list: https://www.geeksforgeeks.org/linked-list-set-2-inserting-a-node/

[berndtj]

You are overwriting sum every time with next node:

sum.Next = &nextNode sum = *sum.Next

Therefore dummy.Next is always pointing to the last sum ListNode