Scramble String

Given a string s1, we may represent it as a binary tree by partitioning it to two non-empty substrings recursively.

Below is one possible representation of s1 = "great":

    great
   /    \
  gr    eat
 / \    /  \
g   r  e   at
           / \
          a   t

To scramble the string, we may choose any non-leaf node and swap its two children.

For example, if we choose the node "gr" and swap its two children, it produces a scrambled string "rgeat".

    rgeat
   /    \
  rg    eat
 / \    /  \
r   g  e   at
           / \
          a   t

We say that "rgeat" is a scrambled string of "great".

Similarly, if we continue to swap the children of nodes "eat" and "at", it produces a scrambled string "rgtae".

    rgtae
   /    \
  rg    tae
 / \    /  \
r   g  ta  e
       / \
      t   a

We say that "rgtae" is a scrambled string of "great".

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

Bruteforce solution: for "great" and "rgeat",

There will be a cut point in "great" and "rgeat" to make them scramble string, so we need to recursively check all substring of s1(0, i), s2(0,i) and s1(i), s2(i) also since the string itself may be scrambeld, we need to check s1(0,i), s2(i) and s1(i), s2(0,i).

ex: (g, r) and (reat, geat) also (g, t), (reat, rgea)

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

        //this is to judge when recursion only pass one letter
        if(s1.length()==1 && s2.length()==1){  
            return s1.charAt(0) == s2.charAt(0);  
        }

        char[] cArray1 = s1.toCharArray();
        char[] cArray2 = s2.toCharArray();
        Arrays.sort(cArray1);
        Arrays.sort(cArray2);

        if (!String.valueOf(cArray1).equals(String.valueOf(cArray2))) {
            return false;
        }

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

            //test front part
            if (isScramble(s11,s21) && isScramble(s12, s22)) {
                return true;
            }

            //test the back part
            s21 = s2.substring(s2.length()-i);
            s22 = s2.substring(0, s2.length()-i);
            if (isScramble(s11, s21) && isScramble(s12, s22)) {
                return true;
            }
        }     
        return false;
    }