int arr[6] = {1, 2, 3, 4, 5, 6};
for (int i = 0; i < 6; i += 2) {
    arr[i] = arr[i] + arr[i+1];
    arr[i+1] = arr[i] - arr[i+1];
    arr[i] = arr[i] - arr[i+1];
}