Java > Array-3 > seriesUp (CodingBat Solution)

Problem:

Given n>=0, create an array with the pattern {1, 1, 2, 1, 2, 3, ... 1, 2, 3 .. n} (spaces added to show the grouping). Note that the length of the array will be 1 + 2 + 3 ... + n, which is known to sum to exactly n*(n + 1)/2.

seriesUp(3) → {1, 1, 2, 1, 2, 3}
seriesUp(4) → {1, 1, 2, 1, 2, 3, 1, 2, 3, 4}
seriesUp(2) → {1, 1, 2}

Solution:

public int[] seriesUp(int n) {
  int[] result = new int[n * (n + 1) / 2];
  int i = 0;
  for (int j = 1; j <= n; ++j)
    for (int k = 1; k <= j; ++k)
      result[i++] = k;
  return result;
}