leetcode——92. 反转链表 II

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

class Solution(object):
    def reverseBetween(self, head, m, n):
        """
        :type head: ListNode
        :type m: int
        :type n: int
        :rtype: ListNode
        """
        a=ListNode(0)
        a.next=head
        q=a
        p=head
        s=1
        stack=[]
        while s<m:
            p=p.next
            q=q.next
            s+=1
        while s>=m:
            stack.append(p)
            p=p.next
            s+=1
            if s==n+1:break
        while stack:
            q.next=stack.pop()
            q=q.next