int binarySearch (int *a, int n, int x) {
    int i = 0, j = n - 1;
    while (i <= j) {
        int k = (i + j) / 2;
        if (a[k] == x) { return k; }
        else if (a[k] < x) { i = k + 1; }
        else { j = k - 1; }
    }
    return -1;
}