Merge k sorted lists

merge_k_sorted_lists.py

# 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]]) -> List:
        out_list = []
        for l in lists:
            while l is not None:
                out_list.append(l.val)
                l = l.next
        # sort it in place
        out_list.sort()
        return out_list