Leetcode 160. Intersection of two linked lists

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.

Notes:

• If the two linked lists have no intersection at all, return null.
• The linked lists must retain their original structure after the function returns.
• You may assume there are no cycles anywhere in the entire linked structure.
• Your code should preferably run in O(n) time and use only O(1) memory.

 class Solution(object):
     def getIntersectionNode(self, headA, headB):
         """
         :type head1, head1: ListNode
         :rtype: ListNode
         """

         n = self.getLength(headA)
         m = self.getLength(headB)

         if n < m:
             return self.getIntersectionNode(headB, headA)

         for i in range(n-m):
             headA = headA.next

         while headA and headB:
             if id(headA) == id(headB):
                 return headA
             headA = headA.next
             headB = headB.next
         return None

     def getLength(self, head):
         n = 0
         while head:
             head = head.next
             n += 1
         return n

1. 总是先操作长度较短的那个list. 

2. 用id(headA) == id(headB) 判断两个node是不是一样