快速找到链表中间的位置

def (head):
    p1 = p2 = head
    if not head or not head.next:
        return head
    while p2.next.next:
        p1 = p1.next
        p2 = p2.next.next
        if not p2.next:
            break
    return p1
ll = creat_ll([1,2,3])
print_ll(find_median(ll))