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.

Credits:
Special thanks to @stellari for adding this problem and creating all test cases.

class Solution(object):
    def getIntersectionNode(self, headA, headB):
        """
        :type head1, head1: ListNode
        :rtype: ListNode
        """
        if headA is None or headB is None:
            return None
        la,lb = 0,0
        starta,startb = headA,headB
        while starta:
            la += 1
            starta = starta.next
        while startb:
            lb += 1
            startb = startb.next
        if lb>la:
            temp = headB
            headB = headA
            headA = temp
            temp = lb
            lb = la
            la = temp
        while la>lb:
            headA = headA.next
            la -= 1
        for i in range(lb):
            if headA==headB:
                return headA
            headB = headB.next
            headA = headA.next
        return None