278. First Bad Version

func firstBadVersion(_ n : Int) -> Int{
    var start = 0
    var end = n
    while start + 1 < end{
        let mid = start + (end - start) / 2
        if isBad(mid){
            end = mid
        }else {
            start = mid
        }
    }
    return isBad(start) ? start : end
}