/**
 * 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) {
        ListNode *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;
    }
};