class Solution {
public:
    ListNode *findFirstCommonNode(ListNode *headA, ListNode *headB) {
        auto p=headA,q=headB;
        while(p!=q)
        {
            if(p) p=p->next;
            else p=headB;
            if(q) q=q->next;
            else q=headA;
        }
        return p;
    }
};