思路：二分法，时间复杂度o(logn)

class Solution(object):

    def nextGreatestLetter(self, letters, target):

        """

        :type letters: List[str]

        :type target: str

        :rtype: str

        """

        left, right = 0, len(letters) - 1

        while left <= right:

            mid = (left + right) / 2

            if letters[mid] <= target:

                left = mid + 1

            else:

                if mid < 1 or (mid >= 1 and letters[mid-1] <= target):

                    return letters[mid]

                right = mid - 1

        return letters[0]