class Solution:

     def FindFirstCommonNode(self, pHead1, pHead2):

         # write code here

         if pHead1==pHead2:

             return pHead1

         p1 = pHead1

         p2 = pHead2

         flag=1

         while p1 and p2 !=None:

             p1 = p1.next

             p2 = p2.next

         if p1 == None:

             flag=2

             p1 = p2

         k=0

         while p1:   #計算兩個鏈表的長度差

             k+=1

             p1=p1.next

         if flag==1:  #flag記錄長的那個鏈表

             while k>0:

                 pHead1=pHead1.next

                 k-=1    #先走k步

         else:

             while k >0:

                 pHead2=pHead2.next

                 k-=1

         while pHead1 and pHead2:

             if pHead1==pHead2:

                 return pHead1

             pHead1=pHead1.next

             pHead2=pHead2.next

         return None