package main

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
// (3->2->1) + (2->1) = (5->3->1), 123 + 12 = 135
func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
	// get head of result list
	head := &ListNode{(l1.Val + l2.Val) % 10, nil}
	remain := int((l1.Val + l2.Val) / 10)
	curr := head
	l1 = l1.Next
	l2 = l2.Next
	// while l1 != [] and l2 != [], calculate l1 + l2
	for l1 != nil && l2 != nil {
		curr.Next = &ListNode{(l1.Val + l2.Val + remain) % 10, nil}
		remain = int((l1.Val + l2.Val + remain) / 10)
		curr = curr.Next
		l1 = l1.Next
		l2 = l2.Next
	}
	// l1 / l2 is nil
	if l1 != nil {
		curr.Next = &ListNode{(l1.Val + remain) % 10, l1.Next}
		remain = int((l1.Val + remain) / 10)
	} else if l2 != nil {
		curr.Next = &ListNode{(l2.Val + remain) % 10, l2.Next}
		remain = int((l2.Val + remain) / 10)
	}
	curr = curr.Next
	// add remainder
	for remain != 0 {
		if curr.Next != nil {
			curr.Next.Val += remain
			remain = int(curr.Next.Val / 10)
			curr.Next.Val %= 10
			curr = curr.Next
		} else {
			curr.Next = &ListNode{remain, nil}
			remain = 0
		}
	}
	return head
}