LeetCode 744. Find Smallest Letter Greater Than Target（寻找比目标字母大的最小字母）

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'。

示例:

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 left = 0, right = letters.length - 1;

        while(left <= right) {
            int mid = left + (right - left) / 2;
            
            if(letters[mid] <= target) {
                left = mid + 1;
            }
            else if(letters[mid] > target) {
                //判断是否是第一个/最小 的比目标大的
                if(mid == 0 || letters[mid - 1] <= target) {
                    return letters[mid];
                }
                else {
                //从右向左逼近
                    right = mid - 1;
                }
            }
        }

        //如果找不到，就返回第一个
        return letters[0];
    }