Find the missing letter

charSet = set(chars)  #去重复
charSet = sorted(charSet) #排序

charSet = ''.join(charSet) #转换成字符串
flag = str.islower(charSet)
charSet = str.lower(charSet)
allChars = []
for i in range(97, 123):
    allChars.append(chr(i))
firstWord = charSet[0]
lastWord = charSet[-1]

firstPosition = allChars.index(firstWord)
lastPosition = allChars.index(lastWord)
allChars = allChars[firstPosition:lastPosition + 1]
for i in range(0, len(allChars)):
    if allChars[i] not in charSet:
        if flag:

            return allChars[i]
        else:
            return str.upper(allChars[i])