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

Merge all the linked-lists into one sorted linked-list and return it.

Example 1:

• Input: lists = [[1,4,5],[1,3,4],[2,6]]
• Output: [1,1,2,3,4,4,5,6]
• Explanation: The linked-lists are:

  [
  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 <= 104
• 0 <= lists[i].length <= 500
• -104 <= lists[i][j] <= 104
• lists[i] is sorted in ascending order.
• The sum of lists[i].length will not exceed 104.