offer-37两个链表是否相交，找第一个公共节点

# coding=utf-8
'''
两个链表是否相交，找第一个公共节点，必知两个链表相交则尾部一定相等，所以长链表先走两个链表长度之差的长度，然后两个指针再一起走
'''
class ListNode():
def __init__(self,x):
self.val=x
self.next=None

def findcommon(p1,p2):
    n1=countlength(p1)
    n2=countlength(p2)
if n1<=0 or n2<=0:
return 0
    phead1=p1
    phead2=p2
if n1>=n2:
for i in range(n1-n2):
            phead1=phead1.next
else:
for i in range(n2-n1):
            phead2=phead2.next
while phead1!=None and phead2!=None:
        phead1=phead1.next
        phead2=phead2.next
if phead1==phead2:
return phead1

def countlength(p):
if p==None:
return -1
    phead=p
    leng=0
    while phead.next!=None:
        phead=phead.next
        leng+=1
    return leng
if __name__ == '__main__':
    node1=ListNode(0)
    node2=ListNode(3)
    node3=ListNode(4)
    node4=ListNode(5)
    node1.next=node2
    node2.next=node3
    node3.next=node4
    node5=ListNode(1)
    node6=ListNode(2)
    node7=ListNode(3)
    node8=ListNode(4)
    node9=ListNode(5)
    node5.next=node6
    node6.next=node7
    node7.next=node8
    node8.next=node9
print(findcommon(node1,node5))