Intersection of Two Linked Lists

Problem:

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.

Solution:
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:

    int length (ListNode *head) {
        int result = 0;
        while (head != NULL) {
            head = head -> next;
            result++;
        }
        return result;
    }

    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        ListNode *temp1, *temp2;
        int listALength = length(headA);
        int listBLength = length(headB);
        
        int bigger, smaller;
        
        if (listALength > listBLength) {
            temp1 = headA;
            temp2 = headB;
            bigger = listALength;
            smaller = listBLength;
        } else {
            temp1 = headB;
            temp2 = headA;
            bigger = listBLength;
            smaller = listALength;
        }
        
        for (int i = bigger; i > smaller; i--) {
            temp1 = temp1->next;
        }
        
        while (temp1 != NULL || temp2 != NULL) {
            if (temp1 == temp2)
                return temp1;
            else {
                temp1 = temp1 -> next;
                temp2 = temp2 -> next;
            }
        }
        return NULL;
    }
};