Leetcode: Reverse Linked List II @Python

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

class Solution:
    # @param head, a ListNode
    # @param m, an integer
    # @param n, an integer
    # @return a ListNode
    def reverseBetween(self, head, m, n):
        if not head: return None
        before=ListNode(0)
        before.next=head
        fast=slow=head
        for i in range(m-1):
            before=slow
            slow=slow.next
        for i in range(n-1):
            fast=fast.next
        for i in range(n-m):
            before.next=slow.next
            slow.next=fast.next
            fast.next=slow
            slow=before.next
        return before.next if m==1 else head