問題：

給定一個升序字元陣列。

求比給定target字元大的，最小的字元。（假設給定字元陣列是迴圈的，即第一個字元>最後一個字元）

Examples:
Input:
letters = ["c", "f", "j"]
target = "a"
Output: "c"

Input:
letters = ["c", "f", "j"]
target = "c"
Output: "f"

Input:
letters = ["c", "f", "j"]
target = "d"
Output: "f"

Input:
letters = ["c", "f", "j"]
target = "g"
Output: "j"

Input:
letters = ["c", "f", "j"]
target = "j"
Output: "c"

Input:
letters = ["c", "f", "j"]
target = "k"
Output: "c"

Note:
letters has a length in range [2, 10000].
letters consists of lowercase letters, and contains at least 2 unique letters.
target is a lowercase letter.

解法：二分查詢(Binary Search)

在給定字元陣列letters中，找到第一個>target字元的字元。

⚠️ 注意：（字元陣列是迴圈：）若未找到，則返回letters[0]

程式碼參考：

 1 class Solution {
 2 public:
 3     char nextGreatestLetter(vector<char>& letters, char target) {
 4         int l = 0, r = letters.size();
 5         while(l<r) {
 6             int
 
 m = l+(r-l)/2;
 7             if(letters[m]>target) {//找到第一個>target
 8                 r = m;
 9             } else {
10                 l = m+1;
11             }
12         }
13         return (l==letters.size())?letters[0]:letters[l];
14     }
15 };