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  1. # Merge K Sorted Lists (LC version 3.04).
  2.  
  3. from heapq import heappush, heappop
  4.  
  5. # List.
  6.  
  7. class ListNode:
  8.     def __init__(self, val=None, next=None):
  9.         self.val = val
  10.         self.next = next
  11.  
  12. def makeList(seq):
  13.     n = None
  14.     for x in reversed(seq):
  15.         n = ListNode(x, n)
  16.     return n
  17.  
  18. def printList(n):
  19.     a = []
  20.     while n:
  21.         a.append(n.val)
  22.         n = n.next
  23.     print('List(', a, ')', sep='')
  24.  
  25. # Merge.
  26.  
  27. def mergeKLists(lists):
  28.     # Push list heads to heap keyed on node.val and a secondary integer.
  29.     # The secondary keys serve a dual purpose:
  30.     #
  31.     #  1. The unique keys prevent the comparison from reaching the node
  32.     #     element which is not comparable.
  33.     #  2. The keys are ordered ascending with respect to the list input
  34.     #     ensuring stability for equal valued elements.
  35.  
  36.     heap = []
  37.     for k, n in enumerate(lists):
  38.         if n:
  39.             heappush(heap, (n.val, k, n))
  40.  
  41.     if not heap:
  42.         return None
  43.  
  44.     # The heap maintains the smallest valued head node for all lists at
  45.     # its front. The node is appended to the result and heap is updated
  46.     # with its next node as the new list head. The loop exits when only
  47.     # a single list remains.
  48.  
  49.     temp = ListNode()
  50.     t = temp
  51.     while True:
  52.         _, k, n = heappop(heap)
  53.         t.next = n
  54.         if not heap:
  55.             return temp.next
  56.         t = t.next
  57.         n = n.next
  58.         if n:
  59.             heappush(heap, (n.val, k, n))
  60.  
  61. # Show.
  62.  
  63. l0 = makeList([])
  64. l1 = makeList([1, 3, 5, 7])
  65. l2 = makeList([2, 4, 6])
  66. l3 = makeList([0, 4, 8])
  67.  
  68. printList(l0)
  69. printList(l1)
  70. printList(l2)
  71. printList(l3)
  72.  
  73. printList(mergeKLists([]))
  74. printList(mergeKLists([l0]))
  75. printList(mergeKLists([l1]))
  76. printList(mergeKLists([l1, l2, l3]))
Success #stdin #stdout 0.09s 14040KB
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List([])
List([1, 3, 5, 7])
List([2, 4, 6])
List([0, 4, 8])
List([])
List([])
List([1, 3, 5, 7])
List([0, 1, 2, 3, 4, 4, 5, 6, 7, 8])
https://ideone.com/ZuC0JP
language:
Python 3 (python  3.12)
created:
12 days ago
visibility:
public

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