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=Nonedef 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 phead1def countlength(p):    if p==None:        return -1    phead=p    leng=0    while phead.next!=None:        phead=phead.next        leng+=1    return lengif __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))