6174是黑洞数（亦称为卡布列克常数，任何非四位相同数字，只要将数字重新排列，组合成最大的值和最小的数再加以相减，重复以上步骤，七次以内就会出现6174。例如：8045，8540-0458=8082，8820-0288=8532，8532-2358=6174。）

6174.c

#include <stdio.h>
#include <stdlib.h>

//------Common base------
typedef int bool;
enum { false, true };
//------Define a struct of four digit number------
typedef struct {
  int thousand;
  int hundred;
  int ten;
  int one;
} Number;
//------Exchange the value of two number------
void exchange(int *a, int *b) {
  int temp = *a;
  *a = *b;
  *b = temp;
}
//------Fill the values of array to struct Number------
void fill(Number *num, int *arr) {
  num->thousand = arr[0];
  num->hundred = arr[1];
  num->ten = arr[2];
  num->one = arr[3];
}
//------Bubble Sort------
void bubbleSort(int *arr, int size, bool (*predicate)(int, int)) {
  for(int i=0; i<size; i++) {
    int *prev = arr+i;
    for(int j=i+1; j<size; j++) {
      int *current = arr+j;
      if((*predicate)(*prev, *current) == true) exchange(prev, current);
    }
  }
}
//------If a is less than b, then the return value is true------
bool less(int a, int b) {
  if(a<b) return true;
  else return false;
}
//------If a is greater than or equal to b, then the return value is true------
bool greaterEqual(int a, int b) {
  if(less(a,b)==true) return false;
  else return true;
}
//------Substract two number and return the result------
int diff(Number *arr) {
  int bigger = arr->thousand*1000 + arr->hundred*100 + arr->ten*10 + arr->one;
  int smaller = (arr+1)->thousand*1000 + (arr+1)->hundred*100 + (arr+1)->ten*10 + (arr+1)->one;
  int result = bigger-smaller;

  printf("%d - %d = %d\n", bigger, smaller, result);
  return result;
}
//------Reorder orignal number to get two number while one is biggest and other smallest------
//------ and then return the result of substraction------
int get(Number ori) {
  int num[4] = {ori.thousand, ori.hundred, ori.ten, ori.one};

  Number ret[2];
  bubbleSort(num, 4, less);
  fill(ret, num);
  bubbleSort(num, 4, greaterEqual);
  fill(ret+1, num);
  return diff(ret);
}
//------Give a number to see whether it can be computed to be 6174 in limited trials------
bool compute(int num, int cnt) {
  if(cnt==0) return false;

  Number n;
  int rem = 0;
  n.thousand = num/1000;
  rem = num%1000;
  n.hundred = rem/100;
  rem = rem%100;
  n.ten = rem/10;
  rem = rem%10;
  n.one = rem;

  if(n.thousand==n.hundred && n.hundred==n.ten && n.ten==n.one) return 0;
  int newNum = get(n);
  if(newNum == 6174) return true;
  else return compute(newNum, cnt-1);
}
//------Constant------
int DEFAULT_CNT = 7;
//------Entry method------
int main(int argc, char **argv) {
  if(argc<2) {
    printf("No input\n");
    return 0;
  }

  for(int i=1; i<argc; i++) {
    printf("Result: %d\n", compute(atoi(argv[i]), DEFAULT_CNT));
  }

  return 0;
}
//------Usage------
//gcc -Wall 6174.c -o 6174
//./6174 2000 1234 #To compute 2000 and 1234