• Source 
    1. // We have triangle made of blocks.
    2. // The topmost row has 1 block, the next row down has 2 blocks, 
    3. // the next row has 3 blocks, and so on. Compute recursively (no loops or multiplication)
    4. // the total number of blocks in such a triangle with the given number of rows. 
    5. //
    6. // triangle(0) → 0
    7. // triangle(1) → 1
    8. // triangle(2) → 3
    9.  
    10. #include <stdio.h>
    11.  
    12. // To enable debug messages uncomment #define
    13. #define TEST 1
    14.  
    15. int triangle(int rows);
    16. void startTesting();
    17.  
    18. int main(void) {
    19. #ifdef TEST
    20.     startTesting();
    21. #endif
    22.  
    23. 	return 0;
    24. }
    25.  
    26. int triangle(int rows) {
    27.   if (rows == 0) {
    28.       return 0;
    29.   }
    30.  
    31.   if (rows == 1) {
    32.       return 1;
    33.   }
    34.  
    35.   return rows + triangle(rows - 1);
    36. } 
    37.  
    38. void startTesting()
    39. {
    40.     int result = 0;
    41.     int i = 0;
    42.  
    43.     for (i = 0; i <= 5; i++) {
    44.         result = triangle(i);
    45.  
    46.          printf("Triangle(%d) = %d\n", i, result);
    47.     }
    48. }