Write a program to merge k sorted singly linked lists and return it as one sorted list.

Example 1

Input:
[
  1->7->9,
  1->3->4,
  4->6
]
Output: 1->1->3->4->4->6->7->9
Explanation: Here k = 3. when we merge the above lists, we get:
1->7->9->1->3->4->4->6
When we sort this unified list, we get the following output:
1->1->3->4->4->6->7->9

Example 2

Input: 
[
   1,
   1->2,
   1->2->3 
] 
Output: 1->1->1->2->2->3
Explanation: Here k = 3. when we merge the above lists, we get:
1->1->2->1->2->3
When we sort this unified list, we get the following output: 
1->1->1->2->2->3