Intersection of Two Linked Lists

Example 1:

Input: 
intersectVal = 8, listA = [4,1,8,4,5], listB = [5,0,1,8,4,5], skipA = 2, skipB = 3

Output:
 Reference of the node with value = 8

Input Explanation:
 The intersected node's value is 8 (note that this must not be 0 if the two lists intersect). From the head of A, it reads as [4,1,8,4,5]. From the head of B, it reads as [5,0,1,8,4,5]. There are 2 nodes before the intersected node in A; There are 3 nodes before the intersected node in B.

Example 2:

Input: 
intersectVal = 2, listA = [0,9,1,2,4], listB = [3,2,4], skipA = 3, skipB = 1

Output:
 Reference of the node with value = 2

Input Explanation:
 The intersected node's value is 2 (note that this must not be 0 if the two lists intersect). From the head of A, it reads as [0,9,1,2,4]. From the head of B, it reads as [3,2,4]. There are 3 nodes before the intersected node in A; There are 1 node before the intersected node in B.

Example 3:

Input: 
intersectVal = 0, listA = [2,6,4], listB = [1,5], skipA = 3, skipB = 2

Output:
 null

Input Explanation:
 From the head of A, it reads as [2,6,4]. From the head of B, it reads as [1,5]. Since the two lists do not intersect, intersectVal must be 0, while skipA and skipB can be arbitrary values.

Explanation:
 The two lists do not intersect, so return null.

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def getIntersectionNode(self, headA, headB):
        """
        :type head1, head1: ListNode
        :rtype: ListNode
        """
#         if not headA or not headB:
#             return None

#         endB = headB
#         while endB.next:
#             endB = endB.next

#         endB.next = headB

#         slow = quick = headA

#         while quick and quick.next:
#             slow = slow.next
#             quick = quick.next.next
#             if slow == quick:
#                 break


#         if not quick or not quick.next:
#             endB.next = None
#             return None

#         slow = headA
#         while slow and slow != quick:
#             slow = slow.next
#             quick = quick.next
#         endB.next = None
#         return slow


        pa, pb = headA, headB

        while pa != pb:
            pa = headB if not pa else pa.next
            pb = headA if not pb else pb.next

        return pa