翻轉一個鏈表

樣例：給出一個鏈表1->2->3->null，這個翻轉后的鏈表為3->2->1->null

一種比較簡單的方法是用“摘除法”。就是先新建一個空節點，然后遍歷整個鏈表，依次令遍歷到的節點指向新建鏈表的頭節點。

那樣例來說，步驟是這樣的：

1. 新建空節點：None
2. 1->None
3. 2->1->None
4. 3->2->1->None

代碼就非常簡單了：

""" Definition of ListNode  class ListNode(object):   def __init__(self, val, next=None):   self.val = val   self.next = next """ class Solution:  """  @param head: The first node of the linked list.  @return: You should return the head of the reversed linked list.      Reverse it in-place.  """  def reverse(self, head):   temp = None   while head:    cur = head.next    head.next = temp    temp = head    head = cur   return temp   # write your code here 

當然，還有一種稍微難度大一點的解法。我們可以對鏈表中節點依次摘鏈和鏈接的方法寫出原地翻轉的代碼：

""" Definition of ListNode  class ListNode(object):   def __init__(self, val, next=None):   self.val = val   self.next = next """ class Solution:  """  @param head: The first node of the linked list.  @return: You should return the head of the reversed linked list.      Reverse it in-place.  """  def reverse(self, head):   if head is None:    return head   dummy = ListNode(-1)   dummy.next = head   pre, cur = head, head.next   while cur:    temp = cur    # 把摘鏈的地方連起來    pre.next = cur.next    cur = pre.next    temp.next = dummy.next    dummy.next = temp   return dummy.next   # write your code here 

需要注意的是，做摘鏈的時候，不要忘了把摘除的地方再連起來

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