/**
 * 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) {
        //让p,q为两链表的头结点，一直走到尾部，之后重新回到对方头部，当他们两个相遇是恰好是公共节点
        //时间复杂度On
        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 q;
    }
};