278. First Bad Version

# The isBadVersion API is already defined for you.
# @param version, an integer
# @return a bool
# def isBadVersion(version):

class Solution(object):
    def firstBadVersion(self, n):
        """
        :type n: int
        :rtype: int
        """
        first, last = 1, n
        while first < last:
            mid = (first + last) // 2
            if isBadVersion(mid):
                last = mid 
            else:
                first = mid + 1
        if first != n + 1 and isBadVersion(first):
            return first
        return -1