leetcode-solutions/21.py at main · juneguo257/leetcode-solutions · GitHub

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# 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]:
        # 1 or more empty linked lists
        if list1 == None:
            return list2
        elif list2 == None:
            return list1

        if (list1.val <= list2.val):
            newHead = list1
            list1 = list1.next
        else:
            newHead = list2
            list2 = list2.next

        curNode = newHead
        while (list1 != None or list2 != None):
            if list1 == None:
                curNode.next = list2
                list2 = list2.next
            elif list2 == None:
                curNode.next = list1
                list1 = list1.next
            elif (list1.val <= list2.val):
                curNode.next = list1
                list1 = list1.next
            else:
                curNode.next = list2
                list2 = list2.next

            curNode = curNode.next

        return newHead