First Bad Version

You are given versions numbered from 1 through n. Once a version is bad, every later version is also bad. The version-control API has already been queried for you: is_bad[i] is True when version i + 1 is bad and False otherwise.

Implement first_bad_version(is_bad) to return the smallest bad version while looking at as few entries as possible. Your algorithm should use binary search and make O(log n) lookups.

Assume 1 <= n and that at least one version is bad. The answers are monotonic: if version v is bad, every version greater than v is bad.

Example 1:
Input: is_bad = [False, False, False, True, True] (n = 5, bad version 4)
Output: 4

Example 2:
Input: is_bad = [True]
Output: 1