class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None
class Solution:
    # 返回ListNode
    def ReverseList(self, pHead):
        prev = None
        cur = pHead
        while cur:
            nex = cur.next
            cur.next = prev
            prev = cur
            cur = nex
        return prev