def sortLetters(self, chars):
        """
        General sort: O(n lg n)
        But in this case, the possible elements are in a closed set - either lower case or upper case; thus O(n)

        The array is abstracted to | closed set | open set |, and move all lower case letters into closed set.

        :param chars: The letters array you should sort.
        :return: NIL, in-place
        """
        closed = -1
        for ind, val in enumerate(chars):
            if ord(val) in ord('a'):  # ASCII A-Za-z
                continue
            else:
                closed += 1
                chars[ind], chars[closed] = chars[closed], chars[ind]
dataset/ETHPy150Open algorhythms/LintCode/Sort Letters by Case.py/Solution.sortLetters/operator `<` -> `in`