21. Merge Two Sorted Lists

You are given the heads of two sorted linked lists list1 and list2

Example 1:

• Input: list1 = [1,2,4], list2 = [1,3,4]
• Output: [1,1,2,3,4,4]

Example 2:

• Input: list1 = [], list2 = []
• Output: []

Example 3:

• Input: list1 = [], list2 = [0]
• Output: [0]

Constraints:

• The number of nodes in both lists is in the range [0, 50].
• -100 <= Node.val <= 100
• Both list1 and list2 are sorted in non-decreasing order.

Approach

Solution

# 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]:

        if not list1:
            return list2
        if not list2:
            return list1

        if list1.val < list2.val:
            head = list1
            list1 = list1.next
        else:
            head = list2
            list2 = list2.next

        current = head
        while list1 and list2:
            if list1.val < list2.val:
                current.next = list1
                list1 = list1.next
            else:
                current.next = list2
                list2 = list2.next
            current = current.next

        current.next = list1 or list2
        return head

Explanation