def reverseKGroup(self, head, k):
"""
:type head: ListNode
:type k: int
:rtype: ListNode
"""
if not head or k == 1:
return head
res = head
for i in range(k-1):
if not res:
return head
res = res.next
Dummy = ListNode(-1)
Dummy.next = head
pre = Dummy
cur = head
while True:
last = cur
# 先获取last，last最后会走出当前的k个ListNode,最后一次判断last是在第k个Node的时候。
for i in range(k):
if not last:
return Dummy.next
last = last.next
temp = None
end = cur
for i in range(k):
nextN = cur.next
cur.next = temp
temp = cur
cur = nextN
end.next = last
pre.next = temp
#注意pre设置为end，就是反转前的第一个ListNode
pre = end
cur = last
