Just use binary search to find the first bad version. The code is as follows.

1 // Forward declaration of isBadVersion API. 2 bool isBadVersion(int version); 3  4 class Solution { 5 public: 6     int firstBadVersion(int n) { 7         int l = 1, r = n; 8         while (l < r) { 9             int m = l + (r - l) / 2;10             if (isBadVersion(m)) r = m;11             else l = m + 1;12         }13         return l;14     }15 };