1. 题目

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"

2. 变形的二分查找

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