744. 尋找比目標字母大的最小字母

給你一個排序後的字元列表 letters ，列表中只包含小寫英文字母。另給出一個目標字母target，請你尋找在這一有序列表裡比目標字母大的最小字母。
在比較時，字母是依序迴圈出現的。舉個例子：
如果目標字母target = 'z'並且字元列表為letters = ['a', 'b']，則答案返回'a'

示例：

輸入:
letters = ["c", "f", "j"]
target = "a"
輸出: "c"

輸入:
letters = ["c", "f", "j"]
target = "c"
輸出: "f"

輸入:
letters = ["c", "f", "j"]
target = "d"
輸出: "f"

輸入:
letters = ["c", "f", "j"]
target = "g"
輸出: "j"

輸入:
letters = ["c", "f", "j"]
target = "j"
輸出: "c"

輸入:
letters = ["c", "f", "j"]
target = "k"
輸出: "c"
 

題解

語言：C++
思路：二分法

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