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.

https://leetcode.com/problems/intersection-of-two-linked-lists/

 1 package leetcode;
 2 public class Solution
 3 {
 4     public static ListNode getIntersectionNode(ListNode headA, ListNode headB)
 5     {
 6         int lenA=len(headA);
 7         int lenB=len(headB);
 8         while(lenA>lenB)
 9         {
10             headA=headA.next;
11             lenA--;
12         }
13         while(lenA<lenB)
14         {
15             headB=headB.next;
16             lenB--;
17         }
18         while(headA!=headB)
19         {
20             headA=headA.next;
21             headB=headB.next;
22         }
23         return headA;
24     }
25     public static int len(ListNode head)
26     {
27         int cout=0;
28         while(head!=null)
29         {
30             head=head.next;
31             cout++;
32         }
33         return cout;
34     }
35     public static void main(String args[])
36     {
37         ListNode a1=new ListNode(1);
38         ListNode a2=new ListNode(2);
39         ListNode a3=new ListNode(3);
40         ListNode b1=new ListNode(4);
41         ListNode b2=new ListNode(5);
42         ListNode c1=new ListNode(6);
43         ListNode c2=new ListNode(7);
44         ListNode headA=new ListNode(0);
45         ListNode headB=new ListNode(0);
46 
47         headA.next=a1;
48         a1.next=a2;
49         a2.next=a3;
50 //        a3.next=c1;
51         c1.next=c2;
52         headB.next=b1;
53         b1.next=b2;
54         b2.next=c1;
55 //        System.out.println(getIntersectionNode(headA,headB).val);
56     }
57 }