/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public var val: Int
 *     public var next: ListNode?
 *     public init() { self.val = 0; self.next = nil; }
 *     public init(_ val: Int) { self.val = val; self.next = nil; }
 *     public init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next; }
 * }
 */
class Solution {
    func mergeTwoLists(_ list1: ListNode?, _ list2: ListNode?) -> ListNode? {
        var list1 = list1
        var list2 = list2

        let head = ListNode()

        var node: ListNode? = head
        while list1 != nil && list2 != nil {
            if list1!.val < list2!.val {
                node!.next = list1
                node = node!.next

                list1 = list1!.next
            } else {
                node!.next = list2
                node = node!.next

                list2 = list2!.next
            }
        }

        if list1 != nil {
            node!.next = list1
        }
        if list2 != nil {
            node!.next = list2
        }

        return head.next
    }
}