LeetCode 278. First Bad Version

class Solution {
public:
    int firstBadVersion(int n) {  
        int l = 1, r = n;
        while (l < r) {
            int mid = (r - l) / 2 + l;
            if (isBadVersion(mid)) r = mid;
            else l = mid + 1;
        }
        return l;
    }
};