原题链接：744. 寻找比目标字母大的最小字母

solution:        二分+特判

class Solution {
public:
    char nextGreatestLetter(vector<char>& letters, char target) {
        int l = 0, r = letters.size();
        while(l < r){
            int mid = l + r >> 1;
            if(letters[mid]>target) r = mid;
            else l = mid + 1;
        }
        if(l == letters.size()) return letters[0];
        return letters[l];
    }
};