如果p1==p2，则这个节点就是公共节点（注意不是值）
public ListNode FindFirstCommonNode(ListNode pHead1, ListNode pHead2)
    {  
        ListNode p1 = pHead1;
        ListNode p2 = pHead2;
        if(p1==null || p2 ==null)return null;
        while(p1 != p2)
        {  
//记住这里一定是if else，不能省去else，要严谨       
            if(p1 == null)
            {               
                p1 = pHead2;
            }else p1 = p1.next;
         
            if(p2 == null)
            {
                p2 = pHead1;  
            }else p2 = p2.next;
        }
        return p1;        
    }