21. 合并两个有序链表 leetcode

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/**
 * 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 mergeTwoLists(_ l1: ListNode?, _ l2: ListNode?) -> ListNode? {
        if l1?.val == nil && l2?.val == nil{
            return nil
        }
        if l1 == nil {
            return l2
        }else if l2 == nil{
            return l1
        }
        var list:ListNode;
        if (l1?.val)! < (l2?.val)!{
            list = l1!
            list.next = self.mergeTwoLists(l1?.next, l2)
        }else{
            list = l2!
            list.next = self.mergeTwoLists(l1, l2?.next)
        }
        
        
        return list;
    }
}