struct ListNode *getIntersectionNode(struct ListNode *headA, struct ListNode *headB) {
    if(!headA || !headB) return NULL;
    struct ListNode *pa = headA;
    struct ListNode *pb = headB;
    while(pa != pb)
    {
        pa = (pa)? pa->next : headB;
        pb = (pb)? pb->next : headA;
    }
    return pa;
}