Recursion with strings

Question

Hallo,
I have to write a method that get's two String type strings (s1, s2) and returns the length of the the biggest continuous sub matrix in the matrix s2 that's all it's digits are present in matrix s1.
For example: if s1 is "xyz" and s2 is "abxyryxzycx" the method will return 4 because the length of sub matrix "yxzy" is 4 and it's the largest sub matrix of s2 that contains only digits from s1(the sub matrices are: "xy", "yxzy", "x").
I have to write this method with recursion and I'm not allowed to use any strings.
When I'm using the defined methods of the String class I can use only the following one's:
public char charAt (int i)
public int indexOf (int ch)
public int length()
public String substring (int i)
I can's see right now how I use "if" conditions with the String methods I'm allowed to use and I can't see the process of solving the problem(should I use linear search?).
Thank's to everybody that can help.
Edited by: ArikG on Jan 26, 2009 10:38 AM

Answer 1

I suppose I have to use linear search, but I can't use it here because I'm not allowed to use loops.
My idea is that if the linear search in s2 a digit from s1 is found so I use the method: public char charAt(int i) and from this point I don't know what to do, and I don't know if it's possible to use linear search in recursion(I know how to do it with loops) and how to write everything here(I am familiar only with the String length methods because we weren't taught in the course about the other Sting methods)