Write a called function that accepts a bracket string and determines whether the order of the brackets is valid

Title description:

　　

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