# LeetCode 23. Merge k Sorted Lists

## Description

https://leetcode.com/problems/merge-k-sorted-lists/

You are given an array of `k` linked-lists `lists`, each linked-list is sorted in ascending order.

Example 1:

```Input: lists = [[1,4,5],[1,3,4],[2,6]]
Output: [1,1,2,3,4,4,5,6]
[
1->4->5,
1->3->4,
2->6
]
merging them into one sorted list:
1->1->2->3->4->4->5->6
```

Example 2:

```Input: lists = []
Output: []
```

Example 3:

```Input: lists = [[]]
Output: []
```

Constraints:

• `k == lists.length`
• `0 <= k <= 10^4`
• `0 <= lists[i].length <= 500`
• `-10^4 <= lists[i][j] <= 10^4`
• `lists[i]` is sorted in ascending order.
• The sum of `lists[i].length` won’t exceed `10^4`.

merge sort

## Python Solution

``````# 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[ListNode]) -> ListNode:
if not lists:
return None

return self.merge_lists(lists, 0, len(lists) - 1)

def merge_lists(self, lists, start, end):
if start == end:
return lists[start]

mid = (start + end) // 2
left = self.merge_lists(lists, start, mid)
right = self.merge_lists(lists, mid + 1, end)

return self.merge_two_lists(left, right)

current = dummy = ListNode(0)

else: