1、題目：反轉單鏈表或雙鏈表

要求：如果鏈表長度為N，時間復雜度為O（N），額外的空間復雜度為O（1）

反轉單鏈表的思路：

1 → 2 → 3 → 4 → 5

（1）first = head = 1

循環：

　　 temp = head.next 2

head.next = temp.next 1 → 3

temp.next = first 2 →1

first = temp 2 【此時的頭結點是2】

代碼：

class Node:
    def __init__(self,value):
        self.value 
 
= value
        self.next = None
def reverseList(head):
    if not head:
        return head
    first = head
    while head.next:
        temp = head.next
        head.next = head.next.next
        temp.next = first
        first = temp
    return first
head = Node(1)
head.next = Node(2)
head.next.next 
 
= Node(3)
head.next.next.next = Node(4)
head.next.next.next.next = Node(5)
reverseList(head)

反轉雙鏈表的思路：

只要將每個節點的 next 和 last 互換就可以了。

代碼：

class Node:
    def __init__(self,value):
        self.value = value
        self.next = None
def reverseList(head):
    if not head:
        return head

#重點
    while
 
 head:
        head.next , head.last = head.last,head.next
        head = head.last
    return head


head = Node(1)
head.last = None
head.next = Node(2)
head.next.last = head
head.next.next = Node(3)
head.next.next.last = head.next
head.next.next.next = Node(4)
head.next.next.next.last = head.next.next
head.next.next.next.next = Node(5)
head.next.next.next.next.last =  head.next.next.next
reverseList(head)

鏈表問題（3）-----反轉