/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *findFirstCommonNode(ListNode *headA, ListNode *headB) {
        auto p=headA,q=headB;
        int s=0;
        if(!p||!q)return NULL;
        while(true)
        {
            if(p==q)
            return p;
            p=p->next,q=q->next;
            if(p==NULL)p=headB,s++;
            if(q==NULL)q=headA,s++;
            if(s==100)return NULL;
        }
    }
};