leetcode 160-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.


/**
 * 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) {
        int len_A = 0, len_B = 0;
        ListNode *p1, *p2;
        p1 = headA;
        p2 = headB;
        if(!p1 || !p2) {
            return NULL;
        }
        while(p1->next) {
            p1 = p1->next;
            len_A++;
        }
        while(p2->next) {
            p2 = p2->next;
            len_B++;
        }
        if(p1 != p2) {
            return NULL;
        }
        else {
            p1 = headA;
            p2 = headB;
            if(len_A < len_B) {
                ListNode *temp;
                temp = p1;
                p1 = p2;
                p2 = temp;
            }
            int temp = 0;
            while(p1) {
                if(temp == abs(len_A - len_B)) {
                    break;
                }
                p1 = p1->next;
                temp++;
            }
            while(p2) {
                if(p1 == p2) {
                    return p1;
                }
                else {
                    p1 = p1->next;
                    p2 = p2->next;
                }
            }
        }
        return NULL;

    }
};