[LeetCode]题解（python）：025-Reverse Nodes in k-Group

题目来源：

　　https://leetcode.com/problems/reverse-nodes-in-k-group/

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题意分析：

　　这道题目和上一题目类似，输入一个链表和一个整型k。每k个翻转一下。不能更改链表的值。

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题目思路：

　　这道题目为了更加直观，先写一个翻转链表的函数。接下来就是链表操作。

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代码（python）：

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution(object):
    def reverse(self,start,end):
        nhead = ListNode(0)
        nhead.next = start
        while nhead.next != end:
            tmp = start.next
            start.next = tmp.next
            tmp.next = nhead.next
            nhead.next = tmp
        return [end,start]
    def reverseKGroup(self, head, k):
        """
        :type head: ListNode
        :type k: int
        :rtype: ListNode
        """
        ans = ListNode(0)
        if head == None:
            return None
        ans.next = head
        start = ans
        while start.next != None:
            end = start
            i = 0
            while i < k - 1:
                end = end.next
                if end.next == None:
                    return ans.next
                i += 1
            tmp = self.reverse(start.next,end.next)
            start.next = tmp[0]
            start = tmp[1]
        return ans.next

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