First Bad Version — Left Boundary Binary Search

# Python
def firstBadVersion(n):
    lo, hi = 1, n
    while lo < hi:
        mid = lo + (hi-lo)//2
        if isBadVersion(mid): hi = mid   # could be first bad
        else: lo = mid+1                  # first bad is after mid
    return lo

// Java
public int firstBadVersion(int n) {
    int lo=1, hi=n;
    while (lo<hi) {
        int mid=lo+(hi-lo)/2;
        if (isBadVersion(mid)) hi=mid;
        else lo=mid+1;
    }
    return lo;
}

// C++
int firstBadVersion(int n) {
    int lo=1, hi=n;
    while (lo<hi) {
        int mid=lo+(hi-lo)/2;
        if (isBadVersion(mid)) hi=mid;
        else lo=mid+1;
    }
    return lo;
}