First Bad Version - Implementation

Here is the implementation:

function firstBadVersion(n):
    left = 1
    right = n

    while left < right:
        mid = left + (right - left) / 2

        if isBadVersion(mid):
            right = mid
        else:
            left = mid + 1

    return left

Note: use left + (right - left) / 2 instead of (left + right) / 2 to avoid integer overflow. This runs in $O(\log n)$ time with $O(1)$ space (excluding input).