Decode String

class Solution:
    def decodeString(self, s: str) -> str:
        stack = []

        cur_num = 0
        cur_str = ''

        for ch in s:
            if ch == '[':
                stack.append(cur_str)
                stack.append(cur_num)
                cur_num = 0
                cur_str = ''
            elif ch == ']':
                x = stack.pop()
                prev_str = stack.pop()
                cur_str = prev_str + x * cur_str
            elif ch.isdigit():
                cur_num = cur_num * 10 + int(ch)
            else:
                cur_str += ch

        return cur_str