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.

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.

/**
 * 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 i = 0;
        ListNode *p = head;
        while ( p != NULL ) {
            p = p->next;
            i += 1;
        }
        return i;
    }
    ListNode* Nth(ListNode *head, int n) {
        int i = 1;
        ListNode* p = head;
        while ( i <= n-1 ) {p = p->next; i += 1; }
        return p;
    }
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        if ( headA == NULL or headB == NULL ) return NULL;
        int nA = length(headA), nB = length(headB);
        ListNode *pA, *pB;
        if ( nA > nB ) { pA = Nth(headA, nA - nB + 1); pB = headB; }
        else { pB = Nth(headB, nB - nA + 1); pA = headA; }
        while ( pA != pB ) {
            if ( pA == NULL or pB == NULL ) return NULL;
            else { pA = pA->next; pB = pB->next;}
        }
        return pA;
    }
};