思路：二分法。

class Solution(object):
    def nextGreatestLetter(self, letters, target):
        letters = list(set(letters))
        letters.sort()
        if target in letters:
            index = letters.index(target)
            if index < len(letters) - 1:
                return letters[(index % len(letters)) + 1]
            if index == len(letters) - 1:
                return letters[(index + 1) % len(letters)]
        l, r = 0, len(letters) - 1
        while l < r:
            mid = int(l + (r - l) / 2)
            if letters[mid] > target:
                r = mid
            else:
                l = mid + 1
        if l == len(letters) - 1 and letters[-1] < target:
            return letters[(l + 1) % len(letters)]
        else:
            return letters[l % len(letters)]