1721. Swapping Nodes in a Linked List

class Solution(object):
    def swapNodes(self, head, k):
        n=0
        ptr=head
        while ptr!=None:
            n+=1
            ptr=ptr.next
        ptr=head
        c=0
        node1=node2=None
        while ptr!=None:
            c+=1
            if c==k:
                node1=ptr
            if c==(n-k+1):
                node2=ptr
            ptr=ptr.next
        temp=node1.val
        node1.val=node2.val
        node2.val=temp
        return head