写一个类，FindString, 有两个字串，s, subs, 要求使用String的indexOf方法来找到subs在s中的开始位置，然后继续找下一个位置，直到结束。-Write a class, FindString, there are two strings, s, subs, requires the use of String s indexOf method to find the subs in the s of the start location, and then cont