leetcode 92 反转链表

class :
    def reverseBetween(self, head, m, n):
        cur = head
        i = 0
        back = tail = front = pre = None
        while cur:
            if m <= i <= n:
                tmp = cur.next #task 1
                if pre is None:
                    tail = cur #record m
                cur.next = pre #task 2
                pre = cur
                cur = tmp
            else:
                if i == m-1:
                    front = cur #record m-1
                elif i == n+1:
                    back = cur
                cur = cur.next # !!move to the next!!
        if front:
            front.next = pre
            res = head
        else:
            res = pre
        tail.next = back
        return res