744.寻找比目标字母大的最小字母 (Easy)*

class Solution {
public:
    char nextGreatestLetter(vector<char>& letters, char target) {
        int n = letters.size();
        int l = 0, r = n;
        while(l < r) {
            int mid = l + (r - l) / 2;
            if(target >= letters[mid]) {
                l = mid + 1;
            }else {
                r = mid;
            }
        }
        return letters[l % n];
    }
};