# coding=utf-8

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

For example, the following two linked lists:
begin to intersect at node c1.

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.

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 getIntersectionNode(self, headA, headB):
        """
        :type head1, head1: ListNode
        :rtype: ListNode
        """
        hA = 0
        hB = 0
        pA = headA
        pB = headB
        while pA:
            hA += 1
            pA = pA.next
        while pB:
            hB += 1
            pB = pB.next
        if hA < hB:
            n = hB - hA
            while n:
                n -= 1
                headB = headB.next
        else:
            n = hA - hB
            while n:
                n -= 1
                headA = headA.next
        while headA != headB and headA and headB:
            headA = headA.next
            headB = headB.next
        return headA


if __name__ == '__main__':
    head1 = ListNode(1)
    head2 = ListNode(2)
    head3 = ListNode(3)
    head4 = ListNode(4)
    head1.next = head2
    head2.next = head3
    head3.next = head4
    while head1:
        print(head1.val)
        head1 = head1.next