algorithm - Mysterious number 6174 [Kaprekar Theorem] in Java -

in 1949 mathematician d. r. kaprekar, devised process known kaprekar's operation. first choose 4 digit number digits not same (that not 1111, 2222,...). rearrange digits largest , smallest numbers these digits can make. finally, subtract smallest number largest new number, , carry on repeating operation each new number.

let's try out, starting number 2005, digits of last year. maximum number can make these digits 5200, , minimum 0025 or 25 (if 1 or more of digits zero, embed these in left hand side of minimum number)

5200 - 0025 = 5175 7551 - 1557 = 5994 9954 - 4599 = 5355 5553 - 3555 = 1998 9981 - 1899 = 8082 8820 - 0288 = 8532 8532 - 2358 = 6174 

now objective verify theorem & find number of iteration take reach 6174.

can give better algorithm? here code.

public int verifykaprekartheorem(int m) {     if (m <= 1000 || m > 9999) {         return -1;     }     string orginal = string.valueof(m);     int count = 0;     while (true) {         int max = integer.parseint(sortstring(orginal, false));         int min = integer.parseint(sortstring(orginal, true));         count++;         int diff = max - min;         if (diff == 6174) {             break;         }         orginal = string.valueof(diff);     }     return count; }  public static string sortstring(string source, boolean assendingorder) {     char[] original = string.valueof(source).tochararray();     arrays.sort(original);     if (assendingorder) {         return new string(original);     }     char[] dessending = new char[original.length];     (int = original.length - 1; >= 0; i--) {         dessending[i] = original[(original.length - 1) - i];     }     return new string(dessending); } 

and testcase

public void testcase01() {     int actual = verifykaprekartheorem(4321);        assertequals(3, actual); } 

i have change integer string char array sorting again integer conversion.

public int verifykaprekartheorem(int m) {     if (m <= 1000 || m > 9999) {         return -1;     }     int count = 0;     while (true) {         int max = largestnumber(m);         int min = smallestnumber(m);         count++;         m = max - min;         if (m == 6174) {             break;         }     }     return count; } private static int largestnumber(int input) {     int[] numbers = new int[10];     (int = input; != 0; /= 10) {         numbers[i % 10]++;     }     int counter = 0;     int result = 0;     (int = 0; < 10; counter += numbers[i++]) {         result += (int) ((math.pow(10, numbers[i]) * - 1) / 9) * math.pow(10, counter);     }     return result; }  private static int smallestnumber(int input) {     int[] numbers = new int[10];     (int = input; != 0; /= 10) {         numbers[i % 10]++;     }     int counter = 0;     int result = 0;     (int = 9; >= 0; counter += numbers[i--]) {         result += (int) ((math.pow(10, numbers[i]) * - 1) / 9) * math.pow(10, counter);     }     return result; }