add_two_numbers.go

/*
 You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

 You may assume the two numbers do not contain any leading zero, except the number 0 itself.

 Example:

 Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
 Output: 7 -> 0 -> 8
 Explanation: 342 + 465 = 807.
*/

func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
	return sumNodes(l1, l2, 0)
}

func sumNodes(l1 *ListNode, l2 *ListNode, rest int) *ListNode {
	if l1 == nil && l2 == nil {
		if rest > 0 {
			return &ListNode{Val: rest}
		}

		return nil
	}

	if l1 != nil && l2 == nil {
		return sumNodes(l1, &ListNode{Val: rest}, 0)
	}

	if l2 != nil && l1 == nil {
		return sumNodes(l2, &ListNode{Val: rest}, 0)
	}

	node := &ListNode{}

	val := l1.Val + l2.Val + rest
	node.Val = val % 10

	node.Next = sumNodes(l1.Next, l2.Next, val/10)
	return node
}