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

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.

//time:o(n) space:o(1)

 1 public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
 2         if (headA == null || headB == null) {
 3             return null;
 4         }
 5         int lengthA = getLength(headA);
 6         int lengthB = getLength(headB);
 7         ListNode currA = headA;
 
 8         ListNode currB = headB;
 9         if (lengthA > lengthB) {
10             while (lengthA != lengthB) {
11                 currA = currA.next;
12                 lengthA--;
13             }
14         } else {
15             while (lengthB != lengthA){
16                 currB = currB.next ;
17                 lengthB--;
18             }
19         }
20         //now they are the same length
21         while (currA!=currB ){
22             currA = currA.next ;
23             currB = currB.next ;
24         }
25         //out of while, means currA == currB, including both == null
26         return currA ;
27     }
28 
29     private int getLength(ListNode head) {
30         int length = 1;
31         while (head != null) {
32             head = head.next;
33             length++;
34         }
35         return length;
36     }

LC.160. Intersection of Two Linked Lists