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.
"()" => 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