/**
 * 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 a=headA,b=headB;
      while(a!=b)
      {
        if(a)a=a->next;
        else a=headB;
        if(b)b=b->next;
        else b=headA;
      }//寻找距离一定，一头空插入另一头
      return a;
    }
};