对替换空格的python代码的时间复杂度有更严格的限制

def replace_spaces(string, replacement):
    new_string = ""
    for char in string:
        if char == " ":
            new_string += replacement
        else:
            new_string += char
    return new_string

def replace_spaces(string, replacement):
    char_list = list(string)
    for i in range(len(char_list)):
        if char_list[i] == " ":
            char_list[i] = replacement
    new_string = "".join(char_list)
    return new_string