アルゴリズム 体操24 Reverse a LinkedList

Reverse a LinkedList

単一LinkedListの先頭を指定すると、LinkedListを逆順にします。反転したLinkedListの新しいヘッドを返す関数を記述します。

例

Solution

三つのポインターである、previous, current, next を利用して、一回のイテレーションでLinkedListを逆にすることができます。

実装

from __future__ import print_function


class Node:
  def __init__(self, value, next=None):
    self.value = value
    self.next = next

  def print_list(self):
    temp = self
    while temp is not None:
      print(temp.value, end=" ")
      temp = temp.next
    print()


def reverse(head):
  previous, current, next = None, head, None
  while current is not None:
    next = current.next  # temporarily store the next node
    current.next = previous  # reverse the current node
    previous = current  # before we move to the next node, point previous to the current node
    current = next  # move on the next node
  return previous


def main():
  head = Node(2)
  head.next = Node(4)
  head.next.next = Node(6)
  head.next.next.next = Node(8)
  head.next.next.next.next = Node(10)

  print("Nodes of original LinkedList are: ", end='')
  head.print_list()
  result = reverse(head)
  print("Nodes of reversed LinkedList are: ", end='')
  result.print_list()


main()