Решение

class Solution: VersionControl() {
    override fun firstBadVersion(n: Int): Int {
        var lo = 1
        var hi = n
        while (lo < hi) {
            val mid = lo + ((hi - lo) ushr 1)
            if (isBadVersion(mid)) hi = mid else lo = mid + 1
        }
        return lo
    }
}