:type s: str
    :rtype: str

def decode_string(s):
    """
    :type s: str
    :rtype: str
    """
    stack = []; cur_num = 0; cur_string = ''
    for c in s:
        if c == '[':
            stack.append((cur_string, cur_num))
            cur_string = ''
            cur_num = 0
        elif c == ']':
            prev_string, num = stack.pop()
            cur_string = prev_string + num * cur_string
        elif c.isdigit():
            cur_num = cur_num*10 + int(c)
        else:
            cur_string += c
    return cur_string