[Leetcode][python]Find Smallest Letter Greater Than Target/寻找比目标字母大的最小字母

class Solution(object):
    def nextGreatestLetter(self, letters, target):
        """
        :type letters: List[str]
        :type target: str
        :rtype: str
        """
        l = 0
        r = len(letters) - 1
        while l <= r:
            m = l + (r - l) / 2
            if letters[m] <= target:
                l = m + 1
            else:
                r = m - 1
        if l < len(letters):
            return letters[l]
        else:
            return letters[0]