MATH - 5c2 , 6c3... combinations

  public int combinationOf(int n, int r) {

return factorial(n) / factorial(r) / factorial(n - r); // int -> overflow, long }
// ni. 4! = 4 x 3 x 2 x 1
public int factorial(int n ) { if (n <= 1) return 1; return n * factorial(n - 1); }

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