链表_leetcode206

# 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 not head or not head.next:
            return head

        pre = None
        cur = head

        while cur:
            next  = cur.next
            cur.next = pre
            pre = cur
            cur = next


        return pre


# 递归解决
class Solution2(object):
    def reverseList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """

        if not head or not head.next:
            return head

        rhead = self.reverseList(head.next)

        head.next.next = head
        head.next = None

        return rhead