Given the head of a singly linked list and 'N', swap the head with Nth node. Return the head of the new linked list.

class sll(object):
    def __init__(self,val):
        self.value = val
        self.next = None

    def __repr__(self):
        return str(self.value)

class Solution(object):
    def swap_head(self,head,N):
        swap_node = head
        while N>1:
            if not swap_node:
                return None
            swap_node = swap_node.next
            N-=1
        if swap_node.next:
            after_swap_node = swap_node.next.next
            after_head = head.next
            new_head = swap_node.next
            swap_node.next = head
            head.next = after_swap_node
            new_head.next = after_head
            return new_head