code
// Forward declaration of isBadVersion API.
bool isBadVersion(int version);
class Solution {
public:
int firstBadVersion(int n) {
int l = 1;
int r = n;
int m;
while (l <= r) {
m = l + (r - l) / 2;
if (isBadVersion(m)) r = m - 1;
else l = m + 1;
}
return l;
}
};