class Solution {
    
    // T: O(logn)
    public char nextGreatestLetter(char[] letters, char target) {
        int left = 0, right = letters.length - 1, mid;
        
        while (left <= right) {
            mid = (left + right) / 2;
            if (letters[mid] <= target) {
                left = mid + 1;
            } else {
                right = mid - 1;
            }
        }

        return left == letters.length ? letters[0] : letters[left];
    }
}