160.py

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


	For example, the following two linked lists:

	A:          a1 → a2
	                   ↘
	                     c1 → c2 → c3
	                   ↗            
	B:     b1 → b2 → b3
	begin to intersect at node c1.
'''

# 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

       	pa, pb = headA, headB
       	while pa != pb:
       		pa = pa.next if pa is not None else headB
       		pb = pb.next if pb is not None else headA

       	return pa if pa else None