算法-链表翻转

反转一个单链表。

简单直白方法：

class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None


class Solution:
    def reverseList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if head is None:
            return head
        first = None
        second = head
        while True:
            third = second.next
            second.next = first

            if third is None:
                break

            first = second
            second = third
        return second

大神快速方法：

class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None


class Solution:
    def reverseList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        prev = None
        while head:
            curr = head
            head = head.next
            curr.next = prev
            prev = curr

        return prev