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.

Runtime: 72ms

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
12         if(!headA || !headB) return NULL;
13
14         if(getLength(headA) > getLength(headB)){
15             int len = getLength(headA);
16             while(len > getLength(headB)){
17                 headA = headA->next;
18                 len--;
19             }
20         }
21         else{
22             int len = getLength(headB);
23             while(len > getLength(headA)){
24                 headB = headB->next;
25                 len--;
26             }
27         }
28
29         while(headA){
30             if(headA == headB)
31                 return headA;
32             headA = headA->next;
33             headB = headB->next;
34         }
35         return NULL;
36     }
37
38     int getLength(ListNode* head){
39         int result = 0;
40
41         while(head){
42             head = head->next;
43             result++;
44         }
45         return result;
46     }
47 };