/**
 * 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) {
        //定义两个指针p和q
        auto p = headA;
        auto q = headB;             //这样不用分情况讨论，绝对是太妙了！
        while(p!=q){
            if(p!=NULL){
                p = p->next;
            }else{
                p =  headB;
            }

            if(q!=NULL){
                q = q->next;
            }else{
                q = headA;
            }
        }

        //跳出while的时候必须有 p == q
        return q;
    }
};