两个链表的第一个公共结点

class Solution {
public:
    ListNode* FindFirstCommonNode( ListNode *pHead1, ListNode *pHead2) {
        if(!pHead1||!pHead2) return NULL;
           int len1=0,len2=0;
         ListNode* p1=pHead1,*p2=pHead2;
         while(p1!=NULL)
         {
             p1=p1->next;
             len1++;
         }
        while(p2!=NULL)
            {
            p2=p2->next;
            len2++;
        }
        p1=pHead1,p2=pHead2;
        if(len1>len2)
            {
            while(p1&&len1>len2)
               {
                p1=p1->next;
                len1--;
            } 
        }
        else 
        {
          while(p2&&len2>len1)
               {
                p2=p2->next;
                len2--;
            }   
        }
        while(p1&&p2)
            {
            if(p1!=p2)
                {
                p1=p1->next;
                p2=p2->next;
            }
            else return p1;
        }
      return NULL;
        
    }
};