[链表]合并 K 个升序链表

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:

        def merge(root1,root2):
            if not root1 and not root2:
                return None
            elif not root1:
                return root2
            elif not root2:
                return root1
            else:
                dummy = ListNode(-1)
                tail = dummy
                cur1 = root1
                cur2 = root2

                while cur1 and cur2:
                    if cur1.val <= cur2.val:
                        tail.next = cur1
                        cur1 = cur1.next
                        tail = tail.next
                        tail.next = None
                    else:
                        tail.next = cur2
                        cur2 = cur2.next
                        tail = tail.next
                        tail.next = None

                if cur1:
                    tail.next = cur1
                if cur2:
                    tail.next = cur2

                return dummy.next

        res = None
        for head in lists:
            res = merge(res,head)
        return res