def isValid(s) : stack = [] for i in s: t= True if i=="(" or i=="{" or i=="[": stack.append(i) elif i==')' and stack[-1]=="(": stack.pop() elif i=='}' and stack[-1]=="{": stack.pop() elif i==']' and stack[-1]=="[": stack.pop() else: t= False return t print(isValid("()"))