题目链接: https://www.acwing.com/activity/content/problem/content/4299/

/**
 * 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 pa = headA, pb = headB;

        while(pa != pb){//循环终止条件，两个指针指向同一个节点时终止
            if(pa) pa = pa->next;//如果当前节点存在，继续向后遍历
            else pa = headB;//否则，从另一个链表的头开始遍历

            if(pb) pb = pb->next;
            else pb = headA;
        }

        //退出循环时，pa和pb指向第一个公共节点，或者都为NULL,两个链表不存在相同的节点
        return pa;
    }
};