Find Smallest Letter Greater than Target

Given a list of sorted characters letters containing only lowercase letters, and given a target letter target, find the smallest element in the list that is larger than the given target.

Letters also wrap around. For example, if the target is target = 'z' and letters = ['a', 'b'], the answer is 'a'.

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:

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

 1 class Solution {
 2     public char nextGreatestLetter(char[] letters, char target) {
 3         int low = 0, high = letters.length - 1;
 4         char letter = '~';
 5         while (low <= high) {
 6             int mid = low + (high - low) / 2;
 7             if (letters[mid] > target) {
 8                 if (letters[mid] > target) {
 9                     letter = letters[mid];
10                 }
11                 high = mid - 1;
12             } else {
13                 low = mid + 1;
14             }
15         }
16         
17         return letter == '~' ? letters[0] : letter;
18     }
19 }