_0025_Reverse_Nodes_in_k_Group.py

# Definition for singly-linked list.
class ListNode(object):
    def __init__(self, x):
        self.val = x
        self.next = None


class Solution(object):
    def reverseKGroup(self, head, k):
        """
        :type head: ListNode
        :type k: int
        :rtype: ListNode
        """
        if not head or not head.next: return head
        count = 0
        cur = head
        while cur and count != k:
            cur = cur.next
            count += 1
        if count == k:
            cur = self.reverseKGroup(cur, k)
            while count:
                temp = head.next
                head.next = cur
                cur = head
                head = temp
                count -= 1
            head = cur
        return head