private static void carry(int[] ca, int index) {
if (ca[index] == (CHARS.length() - 1)) {
ca[index] = 0;
carry(ca, --index);
} else {
ca[index] = ca[index] + 1;
}
}
/**
* Get a unique string
*
* @return the unique string
*/
private static String getUniqueString() {
char[] buf = new char[NUM_CHARS];
carry(lastString, buf.length - 1);
for (int i = 0; i < buf.length; i++) {
buf[i] = CHARS.charAt(lastString[i]);
}
return new String(buf);
}