Intersection of Two LinkedList -- Leetcode

Question:

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.

Answer:
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
         if(!headA || !headB)return NULL;
       
         ListNode *pA = headA, *pB = headB;
         int num1=0, num2=0;
         while(pA){
             pA = pA->next;
             num1++;
         }
         while(pB){
             pB = pB->next;
             num2++;
         }
       
         pA = headA;
         pB = headB;
         if(num1 >= num2){
             int extra = num1-num2;
             while(extra-- > 0){
                 pA = pA->next;
             }
         }else if(num1 < num2){
             int extra = num2-num1;
             while(extra-- > 0){
                 pB = pB->next;
             }
         }
       
         while(pA && pB && pA!=pB){
                 pA = pA->next;
                 pB = pB->next;
         }
         if(pA && pB){
            return pA;
         }else{
            return NULL;
         }
    }
};