---
# System prepended metadata

title: Leetcode 21. Merge Two Sorted Lists
tags: [Linked List, Leetcode]

---

## 題解

```python=
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
        output = ListNode()
        cur = output
        while list1 and list2:
            if list1.val < list2.val:
                temp = list1.next
                list1.next = None
                cur.next = list1
                list1 = temp
            else:
                temp = list2.next
                list2.next = None
                cur.next = list2
                list2 = temp
            cur = cur.next
        
        while list1:
            temp = list1.next
            list1.next = None
            cur.next = list1
            list1 = temp
            cur = cur.next
        while list2:
            temp = list2.next
            list2.next = None
            cur.next = list2
            list2 = temp
            cur = cur.next
        
        return output.next
```

2023.08.30 AC

```python=
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
        output = ListNode()
        cur = output
        c1 = list1
        c2 = list2
        while c1 and c2:
            if c1.val < c2.val:
                cur.next = c1
                c1 = c1.next
            else:
                cur.next = c2
                c2 = c2.next
            cur = cur.next
        if c1:
            cur.next = c1
        else:
            cur.next = c2
        return output.next
```