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  1. int nextUnit(int u) {
  2.     // implement u * 10 using addition.
  3.     int unit = u + u + u;
  4.     return unit + unit + unit + u; 
  5. }
  6.  
  7. int digitSum(int x) {
  8.     int units[24] = {0};
  9.     units[0] = 1;
  10.     int column = 0;
  11.  
  12.     while (units[column] < x) {
  13.         column++;
  14.         units[column] = nextUnit(units[column - 1]);
  15.     }
  16.  
  17.     int sum = 0;
  18.     while (x > 0) {
  19.         if (x >= units[column]) {
  20.             x -= units[column];
  21.             sum++;
  22.         } else {
  23.             column--;
  24.         }
  25.     }
  26.     return sum;
  27. }
  28.  
  29. int isDivBy9(int x)
  30. {
  31.     if (x == 0) {
  32.         return x == 0;
  33.     }
  34.     if (x < 0) {
  35.         x = -x;
  36.     }
  37.  
  38.     int sum = digitSum(x);
  39.  
  40.     if (sum < 10) {
  41.         return sum == 9;
  42.     }
  43.  
  44.     return isDivBy9(sum);
  45.  
  46. }
  47.  
  48. int main() {
  49.     for (int i=-10000; i < 1000000; ++i) {
  50.         if (isDivBy9(i) != (i%9 == 0)) {
  51.             return 1;
  52.         }
  53.     }
  54.     return 0;
  55. }
  56.  
Success #stdin #stdout 0.39s 2048KB
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https://ideone.com/P1mdfj
language:
C99 (gcc 8.3)
created:
10 years ago
visibility:
public

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