Solve maximum sum subarray of size k using sliding window.

int maxSumK(int[] a, int k) {
  int sum = 0;
  for (int i = 0; i < k; i++) sum += a[i];
  int best = sum;
  for (int i = k; i < a.length; i++) {
    sum += a[i] - a[i - k];
    best = Math.max(best, sum);
  }
  return best;
}