int lo = 0;
 int hi = a.length - 1;
 while (lo <= hi) {
    // Key is in a[lo..hi] or not present.
    int mid = lo + (hi - lo) / 2;
    if      (key < a[mid]) hi = mid - 1;
    else if (key > a[mid]) lo = mid + 1;
    else return mid;
 }
 return -1;