# -*- coding: utf-8 -*-
'''
鏈表逆序
'''
class ListNode: 
  def __init__(self,x): 
    self.val=x
    self.next=None
 
'''
第一種方法：
對于一個長度為n的單鏈表head,用一個大小為n的數組arr儲存從單鏈表從頭
到尾遍歷的所有元素，在從arr尾到頭讀取元素簡歷一個新的單鏈表
時間消耗O(n),空間消耗O(n)
'''   
def reverse_linkedlist1(head):
  if head == None or head.next == None: #邊界條件
    return head
  arr = [] # 空間消耗為n,n為單鏈表的長度
  while head:
    arr.append(head.val)
    head = head.next
  newhead = ListNode(0)
  tmp = newhead
  for i in arr[::-1]:
    tmp.next = ListNode(i)
    tmp = tmp.next
  return newhead.next
 
'''
開始以單鏈表的第一個元素為循環(huán)變量cur,并設置2個輔助變量tmp,保存數據;
newhead,新的翻轉鏈表的表頭。
時間消耗O(n),空間消耗O(1)
'''
 
def reverse_linkedlist2(head):
  if head == None or head.next == None: #邊界條件
    return head
  cur = head #循環(huán)變量
  tmp = None #保存數據的臨時變量
  newhead = None #新的翻轉單鏈表的表頭
  while cur:
    tmp = cur.next
    cur.next = newhead
    newhead = cur  # 更新 新鏈表的表頭
    cur = tmp
  return newhead
   
'''
開始以單鏈表的第二個元素為循環(huán)變量，用2個變量循環(huán)向后操作,并設置1個輔助變量tmp,保存數據;
時間消耗O(n),空間消耗O(1)
'''
 
 
def reverse_linkedlist3(head):
  if head == None or head.next == None: #邊界條件
    return head
  p1 = head #循環(huán)變量1
  p2 = head.next #循環(huán)變量2
  tmp = None #保存數據的臨時變量
  while p2:
    tmp = p2.next
    p2.next = p1
    p1 = p2
    p2 = tmp
  head.next = None
  return p1
 
'''
遞歸操作，先將從第一個點開始翻轉轉換從下一個節(jié)點開始翻轉
直至只剩一個節(jié)點
時間消耗O(n),空間消耗O(1)
'''
 
def reverse_linkedlist4(head):
  if head is None or head.next is None:
    return head
  else:
    newhead=reverse_linkedlist4(head.next)
    head.next.next=head
    head.next=None
  return newhead
 
     
def create_ll(arr):
  pre = ListNode(0)
  tmp = pre
  for i in arr:
    tmp.next = ListNode(i)
    tmp = tmp.next
  return pre.next
   
def print_ll(head):
  tmp = head
  while tmp:
    print tmp.val
    tmp=tmp.next
 
a = create_ll(range(5))
print_ll(a) # 0 1 2 3 4
a = reverse_linkedlist1(a)
print_ll(a) # 4 3 2 1 0
a = reverse_linkedlist2(a)
print_ll(a) # 0 1 2 3 4
a = reverse_linkedlist3(a)
print_ll(a) # 4 3 2 1 0
a = reverse_linkedlist4(a)
print_ll(a) # 0 1 2 3 4