题目大意： 反转一个链表，要求分别用迭代和递归实现 Python代码：
# 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 """ if head is None: return None # head, tail = self.reverseListRecursively(head) head = self.reverseListIteratively(head) return head # 递归 52ms def reverseListRecursively(self, head): if head.next is None: return head, head h, p = self.reverseListRecursively(head.next) p.next = head head.next = None return h, head # 迭代 95ms -> 69ms def reverseListIteratively(self, head): p = head q = p.next if q is None: return p r = q.next p.next = None # 第一次写的迭代 95ms # while q is not None: # q.next = p # p = q # q = r # if r is not None: # r = r.next # 69ms while r is not None: q.next = p p = q q = r r = r.next q.next = p return q代码不难写，but AC后发现迭代的时间（95ms）要大于递归的时间（52ms）。按理说一般认为迭代的效率要优于递归，因为递归是自己调用自己，系统每次调用自己都需要分配空间，并把当前场景压栈，调用结束后还要释放空间，不仅浪费空间还浪费时间。可能是写得太渣了，so优化一下，优化后的时间为69ms，比优化前好些了。
