Leetcode: Intersection of Two Linked Lists

A:          a1 → a2
                   ↘
                     c1 → c2 → c3
                   ↗            
B:     b1 → b2 → b3

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution
{
private:
    int length(ListNode *head)
    {
    	int count = 0;
    	ListNode *p = head;
    	while (p)
    	{
    		count++;
    		p = p->next;
    	}
    	return count;
    }
public:
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB)
    {
    	if (headA == NULL || headB == NULL)
    	{
    		return NULL;
    	}
    
    	int lengthA = length(headA);
    	int lengthB = length(headB);
    	
    	int offLength = lengthA - lengthB;
    	if (offLength < 0)
    	{
    		offLength *= -1;
    	}
    
    	ListNode *pa = headA;
    	ListNode *pb = headB;
    	for (int i = 0; i < offLength; i++)
    	{
    		if (lengthA > lengthB)
    		{
    			pa = pa->next;
    		}
    		else
    		{
    			pb = pb->next;
    		}
    	}
    
    	while (pa != pb)
    	{
    		pa = pa->next;
    		pb = pb->next;
    	}
    	return pa;
    }
};