2. Add Two Numbers

add-two-numbers.go

// https://leetcode.com/problems/add-two-numbers/
//
// Time: O(N)
// Space: O(N)
//
// N = number of nodes in the longer list

// .................... //

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
    head := &ListNode{}
    node := head
    sum := 0

    for l1 != nil || l2 != nil || sum > 0 {
        if l1 != nil {
            sum += l1.Val
            l1 = l1.Next
        }
        if l2 != nil {
            sum += l2.Val
            l2 = l2.Next
        }

        node.Next = &ListNode{Val: sum % 10}
        node = node.Next

        sum /= 10
    }

    return head.Next
}