Write a program to find the node at which the intersection of two singly linked lists begins.

class Solution(object):

    def getIntersectionNode(self, headA, headB):

        """

        :type head1, head1: ListNode

        :rtype: ListNode

        """

        if not headA or not headB:

            return None

        pa = headA

        pb = headB

        while pa is not pb:

            pa = headB if pa is None else pa.next

            pb = headA if pb is None else pb.next

        return pa

1 如果考虑时间复杂度，不用for循环用while循环

2 这里的switch head是关键，可以加快meet的速度