python 算法：Valid Parentheses

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.

Examples
"()"              =>  true
")(()))"          =>  false
"("               =>  false
"(())((()())())"  =>  true
Constraints
0 <= input.length <= 100

Along with opening (() and closing ()) parenthesis, input may contain any valid ASCII characters. Furthermore, the input string may be empty and/or not contain any parentheses at all. Do not treat other forms of brackets as parentheses (e.g. [], {}, <>).

解法：

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