sample1.java

int go(int[] a, int l, int r) {
    if (l == r)
        return a[l];

    int mid = (l + r) / 2;
    return go(a, l, mid) + go(a, mid + 1, r);
}

int[] a = {...};
int N = a.length;

int sum = go(a, 0, N - 1);

sample2.java

private static int go(int[] a, int l, int r) {
    if (l == r)
        return a[l];
    
    int mid = (l + r) / 2;
    int sum = go(a, l, mid) + go(a, mid + 1, r);

    for (int i = l; i <= r; i++)
        sum += a[i];
    
    return sum;
}

int[] a = {5, 7, 9};
int N = a.length;

int result = go(a, 0, N - 1);