2.两数相加

1.逐步迭代加和，需要注意的是最后一次的进位

public class Solution {
    func addTwoNumbers(_ l1: ListNode?, _ l2: ListNode?) -> ListNode?  {
        var l1 = l1
        var l2 = l2
        var head: ListNode? = nil
        var tail: ListNode? = nil
        var carry = 0
        while l1 != nil || l2 != nil {
            let n1 = l1 != nil ? l1!.val : 0
            let n2 = l2 != nil ? l2!.val : 0
            let sum = n1 + n2 + carry
            if head == nil {
                head = ListNode(sum % 10);
                tail = head
            }else {
                tail?.next = ListNode(sum % 10)
                tail = tail?.next
            }
            carry = sum / 10
            if l1 != nil {
                l1 = l1?.next
            }
            
            if l2 != nil {
                l2 = l2?.next
            }
        }
        
        if carry > 0 {
            tail?.next = ListNode(carry)
        }
        
        return head
    }
}