23. Merge k Sorted Lists

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

Example:

Input:
[
1->4->5,
1->3->4,
2->6
]
Output: 1->1->2->3->4->4->5->6

# Definition for singly-linked list.
class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None

class Solution:
    def mergeKLists(self, lists):
        """
        :type lists: List[ListNode]
        :rtype: ListNode
        """
        if len(lists)==0:
            return None
        if len(lists)==1:
            return lists[0]
        head = self.mergeTwoLists(lists[0],lists[1])
        for i in range(2,len(lists)):
            head = self.mergeTwoLists(head,lists[i])
        return head
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if l1 is None and l2 is None:
            return l1
        if l1 is None:
            return l2
        if l2 is None:
            return l1
        start = ListNode(0)
        pos = start
        while l1 and l2:
            if l1.val<l2.val:
                pos.next = l1
                l1 = l1.next
                pos = pos.next
            else:
                pos.next = l2
                l2 = l2.next
                pos = pos.next
        if l1:
            pos.next = l1
            l1 = l1.next
            pos = pos.next
        if l2:
            pos.next = l2
            l2 = l2.next
            pos = pos.next
        return start.next