# Time:  O(n)
# Space: O(1)

class ListNode(object):
    def __init__(self, x):
        self.val = x
        self.next = None

    def __repr__(self):
        if self:
            return "{} -> {}".format(self.val, repr(self.next))
class Solution(object):
    # @param head, a ListNode
    # @param m, an integer
    # @param n, an integer
    # @return a ListNode
    def reverseBetween(self, head, m, n):
        diff, dummy, cur = n - m + 1, ListNode(-1), head
        dummy.next = head

        last_unswapped = dummy
        while cur and m > 1:
            cur, last_unswapped, m = cur.next, cur, m - 1

        prev, first_swapped = last_unswapped,  cur
        while cur and diff > 0:
            cur.next, prev, cur, diff = prev, cur, cur.next, diff - 1

        last_unswapped.next, first_swapped.next = prev, cur

        return dummy.next