exercism

Exercism solutions.
git clone git://code.dwrz.net/exercism
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difference_of_squares.c (617B)

      1 #include <stdio.h>
      2 
      3 // The sum of the first N natural numbers can be calculated as
      4 // N * (N + 1) / 2. To get the square, we multiply the sum with itself.
      5 // This formula saves us from having to loop up to N.
      6 int square_of_sum(int n) {
      7 	return ((n * (n + 1)) / 2) * ((n * (n + 1)) / 2);
      8 }
      9 
     10 // The sum of the squares of the first N natural numbers can be
     11 // calculated as (n * (n + 1) * ((2 * n) + 1)) / 6.
     12 // This formula saves us from having to loop up to N.
     13 int sum_of_squares(int n) {
     14 	return (n * (n + 1) * ((2 * n) + 1)) / 6;
     15 }
     16 
     17 int difference_of_squares(int n) {
     18 	return square_of_sum(n) - sum_of_squares(n);
     19 }