def sortLetters(self, chars):
        closed = -1
        for ind, val in enumerate(chars):
            if ord(val) < ord('a'):  # ASCII A-Za-z
                continue
            else:
                closed += 1
                chars[ind], chars[closed] = chars[closed], chars[ind]
General sort: O(n lg n)
dataset/ETHPy150Open algorhythms/LintCode/Sort Letters by Case.py/Solution.sortLetters