206. 反转链表

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
        if not head :
            return head
        
        pre =None
        cur =head 
        while cur :
            temp =cur.next 
            cur.next=pre
            pre =cur 
            cur =temp
        return pre