# 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.



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

class Solution(object):
    def get_length(self,head):
        temp= head
        length=0
        while temp:
            length+=1
            temp=temp.next
        return length
    def getIntersectionNode(self, headA, headB):
        """
        :type head1, head1: ListNode
        :rtype: ListNode
        """

        M= self.get_length(headA)
        N= self.get_length(headB)
        diff = abs(M-N)
        if diff and M<N:
            headA,headB= headB, headA
        for i in xrange(diff):
            headA=headA.next 

        while headA and headB and headA is not headB:
            headA= headA.next
            headB= headB.next
        return headA

        

