替换字符串中的字符为“(” 或“)”

题目描述：
# The goal of this exercise is to convert a string to a new string where each character
# in the new string is "(" if that character appears only once in the original string,
# or ")" if that character appears more than once in the original string.
# Ignore capitalization when determining if a character is a duplicate

我的解答：

def duplicate_letter(b):
    c = {}
    for i in b:
        if i not in c.keys():
            c[i] = 1
        elif i in c.keys():
            c[i] = c[i] + 1
    new_word = b.lower()
    for j in new_word:
        if c[j] > 1:
            new_word = new_word.replace(j, ")")
        elif c[j] == 1:
            new_word = new_word.replace(j, "(")
    return new_word