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23. Merge k Sorted Lists

Hard

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 <= 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.

Solution

# Definition for singly-linked list.
# class ListNode
#     attr_accessor :val, :next
#     def initialize(val = 0, _next = nil)
#         @val = val
#         @next = _next
#     end
# end
# @param {ListNode[]} lists
# @return {ListNode}
def merge_k_lists(lists)
  return nil if lists.empty?
  merge_k_lists_helper(lists, 0, lists.length)
end

def merge_k_lists_helper(lists, left_index, right_index)
  if right_index > left_index + 1
    mid = (left_index + right_index) / 2
    left = merge_k_lists_helper(lists, left_index, mid)
    right = merge_k_lists_helper(lists, mid, right_index)
    merge_two_lists(left, right)
  else
    lists[left_index]
  end
end

def merge_two_lists(left, right)
  return right if left.nil?
  return left if right.nil?

  res = nil
  if left.val <= right.val
    res = left
    left = left.next
  else
    res = right
    right = right.next
  end

  node = res
  while left || right
    if left.nil?
      node.next = right
      right = right.next
    elsif right.nil?
      node.next = left
      left = left.next
    else
      if left.val <= right.val
        node.next = left
        left = left.next
      else
        node.next = right
        right = right.next
      end
    end
    node = node.next
  end

  res
end

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