206. &92 Reverse Linked List

题目要求：

Reverse a singly linked list.

image.png

# Time:   O(1)
# Space:  O(n)
# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def reverseList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        new_head = None       #指针作用
        next = None
        
        while head:
            next = head.next
            head.next =new_head
            new_head = head
            head = next

        return new_head

补充知识点：Python关于正负无穷float("inf")的用法：

1. Python利用float("inf")和float("inf")表示正负无穷
2. 利用float("inf")做简单加、乘算术运算仍会得到float("inf")：
   '>>>1 + float("inf")
   inf
3. '>>>dummy, next, head = 1, 2, 3
   '>>>dummy, next, head = head, dummy, next
   '>>>print(dummy, next, head)
   '3 1 2

# Time:   O(1)
# Space:  O(n)
# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def reverseList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        dummy = ListNode(float("-inf"))
        while head:
               dummy.next, head.next, head = head, dummy.next, head.next
         return dummy.next

if __name__ == "__main__":
    head = ListNode(1)
    head.next = ListNode(2)
    head.next.next = ListNode(3)
    head.next.next.next = ListNode(4)
    head.next.next.next.next = ListNode(5)
    print(Solution().reverseList(head))

其中：# dummy.next, head.next, head = head, dummy.next, head.next是节点逆序的常用写法！！！
dummy.next 第一次执行的时候为None，所以这么写没毛病。

示意图