编写一个调用的函数，该函数接受一个括号字符串，并确定括号的顺序是否有效

题目描述：

　　

Write a function called that takes a string of parentheses, and determines if the order of the parentheses is valid. 
The function should return true if the string is valid, and false if it's invalid.

我的解答：
　　

def valid_parentheses(string):
    conn = 0
    for i in string:
        if i == '(':
            conn += 1
        if i == ')':
            conn -= 1
        if conn < 0:
            return False
    if conn == 0:
        return True
    else:
        return False