Intersection of Two Linked Lists

Problem

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

Example

The following two linked lists:

    A:          a1 → a2
                       ↘
                         c1 → c2 → c3
                       ↗
    B:     b1 → b2 → b3

begin to intersect at node c1.

Note

• 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.

Challenge

Your code should preferably run in O(n) time and use only O(1) memory.