744. Find Smallest Letter Greater Than Target

    char nextGreatestLetter(vector<char>& letters, char target) {
        if(letters.back() <= target || letters.front()> target) return letters.front();
        int lo = 0, hi = letters.size() - 1, mi;
        while(lo<hi) {
            mi = (lo + hi) /2;
            if(mi == lo) break;
            if(target < letters[mi]) hi = mi;
            else if(target >= letters[mi]) lo = mi;
        }
        return letters[hi];
    }