给定一个只包含小写字母的有序数组letters 和一个目标字母 target，寻找有序数组里面比目标字母大的最小字母。 数组里字母的顺序是循环的。举个例子，如果目标字母target = 'z' 并且

给定一个只包含小写字母的有序数组letters 和一个目标字母 target，寻找有序数组里面比目标字母大的最小字母。

数组里字母的顺序是循环的。举个例子，如果目标字母target = ‘z’ 并且有序数组为 letters = [‘a’, ‘b’]，则答案返回 ‘a’。
输入:
letters = [“c”, “f”, “j”]
target = “a”
输出: “c”

输入:
letters = [“c”, “f”, “j”]
target = “c”
输出: “f”

输入:
letters = [“c”, “f”, “j”]
target = “d”
输出: “f”

输入:
letters = [“c”, “f”, “j”]
target = “g”
输出: “j”

输入:
letters = [“c”, “f”, “j”]
target = “j”
输出: “c”

输入:
letters = [“c”, “f”, “j”]
target = “k”
输出: “c”

public char nextGreatestLetter(char[] letters, char target) {
        int n = letters.length;
        int low = 0, high = n - 1;
        while (low <= high) {
            int mid = (low + high) >> 1;
            if (letters[mid] <= target) {
                low = mid + 1;
            } else {
                high = mid - 1;
            }
        }
        return low < n ? letters[low] : letters[0];
    }