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# 25.合并两个排序的链表

## 题目描述

输入两个递增排序的链表，合并这两个链表并使新链表中的节点仍然是递增排序的。

示例1：

输入：1->2->4, 1->3->4 输出：1->1->2->3->4->4 限制：

0 <= 链表长度 <= 1000

注意：本题与主站 21 题相同：<https://leetcode-cn.com/problems/merge-two-sorted-lists/>

来源：力扣（LeetCode）

链接：<https://leetcode-cn.com/problems/he-bing-liang-ge-pai-xu-de-lian-biao-lcof> 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

## 题解

```swift
func mergeTwoLists(_ l1: ListNode?, _ l2: ListNode?) -> ListNode? {
    var line1 = l1
    var line2 = l2
    var firstNode: ListNode?
    var lastNode: ListNode?

    while let temp1 = line1, let temp2 = line2 {
        let isL1VSmaller = temp1.val < temp2.val
        let node = ListNode(isL1VSmaller ? temp1.val : temp2.val)
        if firstNode == nil {
            firstNode = node
        }
        else {
            lastNode?.next = node
        }
        lastNode = node
        isL1VSmaller ? (line1 = temp1.next) : (line2 = temp2.next)
    }

    if line1 == nil {
        lastNode?.next = line2
    }
    else {
        lastNode?.next = line1
    }

    return firstNode != nil ? firstNode : (l1 != nil ? l1 : l2)
}
```