278.第一个错误的版本

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class Solution {
public:
    int firstBadVersion(int n) {
        int lo = 1,
        ho = n == 2147483647 ? n : n + 1,
        mid ;
        while(lo < ho) {
            mid = lo + (ho - lo) / 2;
            if(isBadVersion(mid)) {
                ho = mid;
            } else if (!isBadVersion(mid))
                lo = mid + 1;
        }
        return lo;
    }
};