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
        """
        l, r = 1, n
        while l <= r:
            mid = (l + r) >> 1
            if isBadVersion(mid):
                if mid == 1 or isBadVersion(mid-1)==False:
                    return mid
                else:
                    r = mid
            else:
                l = mid + 1