class Solution:
    # 返回ListNode
    def ReverseList(self, pHead):
        # write code here
        if not pHead: return None
        else:
            prev = None
            cur = pHead
            while cur:
                #mid = cur.next
                #cur.next = prev
                #prev = cur
                #cur = mid

                # python可以直接这样交换
                cur.next, prev, cur = prev, cur, cur.next
            return prev