def is_balanced(str):
    Stack stack
    for char in str:
        if char in [ '(', '[' ]:
            stack.push(char)
        else:
            if stack.empty(): return False
            top <- stack.pop()
            if (top = '[' and char != ']') or
                (top = '(' and char != ')'):
                    return False
    return stack.empty()