Scramble String(Hard)

Given two strings s1 and s2 of the same length, determine if s2 is a scrambled string of s1.

public boolean isScramble(String s1, String s2) {
    if(s1.length()!=s2.length())
        return false;

    if(s1.length()==0 || s1.equals(s2))
        return true;

    char[] arr1 = s1.toCharArray();
    char[] arr2 = s2.toCharArray();
    Arrays.sort(arr1);
    Arrays.sort(arr2);
    if(!new String(arr1).equals(new String(arr2))){
        return false;
    }

    for(int i=1; i<s1.length(); i++){
        String s11 = s1.substring(0, i);
        String s12 = s1.substring(i, s1.length());
        String s21 = s2.substring(0, i);
        String s22 = s2.substring(i, s2.length());
        String s23 = s2.substring(0, s2.length()-i);
        String s24 = s2.substring(s2.length()-i, s2.length());

        if(isScramble(s11, s21) && isScramble(s12, s22))
            return true;
        if(isScramble(s11, s24) && isScramble(s12, s23))
            return true;    
    }    

    return false;
}