160. Intersection of Two Linked Lists java solutions

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.

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) {
 7  *         val = x;
 8  *         next = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
14         if(headA == null || headB == null) return null;
15         ListNode ca = headA, cb = headB;
16         int cnta = 0, cntb = 0;
17         while(ca != null){
18             cnta++;
19             ca = ca.next;
20         }
21         while(cb != null){
22             cntb++;
23             cb = cb.next;
24         }
25         int sub = Math.abs(cnta - cntb);
26         if(cnta > cntb){
27             while(sub > 0){
28                 headA = headA.next;
29                 sub--;
30             }
31         }else{
32             while(sub > 0){
33                 headB = headB.next;
34                 sub--;
35             }
36         }
37         while(headA != headB){
38             headA = headA.next;
39             headB = headB.next;
40         }
41         return headA;
42     }
43 }