You are given two linked lists representing two non-negative numbers. 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.
Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public var val: Int
 *     public var next: ListNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.next = nil
 *     }
 * }
 */
class Solution {
    func addTwoNumbers(l1: ListNode?, _ l2: ListNode?) -> ListNode? {
        if l1 == nil && l2 == nil {
            return nil
        }
        var g1 = l1
        var g2 = l2
        
        let result = ListNode(0)
        var g3 = result
        
        var move = false
        var carry = false
        while g1 != nil && g2 != nil {
            if move {
                g3.next = ListNode(0)
                g3 = g3.next!
            } else {
                move = true
            }
            
            carry = setNodeAndReturnCarry(g3, val: g1!.val + g2!.val, carry: carry)
            
            g1 = g1!.next
            g2 = g2!.next
        }
        
        if g1 != nil {
            while g1 != nil {
                if move {
                    g3.next = ListNode(0)
                    g3 = g3.next!
                } else {
                    move = true
                }
                
                carry = setNodeAndReturnCarry(g3, val: g1!.val, carry: carry)
                
                g1 = g1!.next
            }
        } else {
            while g2 != nil {
                if move {
                    g3.next = ListNode(0)
                    g3 = g3.next!
                } else {
                    move = true
                }
                
                carry = setNodeAndReturnCarry(g3, val: g2!.val, carry: carry)
                
                g2 = g2!.next
            }
        }
    
        if carry {
            g3.next = ListNode(1)
        }
        
        return result
    }
    
    func setNodeAndReturnCarry(g: ListNode, val: Int, carry: Bool) -> Bool {
        g.val = val
        var tmpCarry = carry
        
        if tmpCarry {
            g.val += 1
        }
        
        if g.val >= 10 {
            g.val = g.val%10
            tmpCarry = true
        } else {
            tmpCarry = false
        }
        
        return tmpCarry
    }
}