class Solution:
    def decodeString(self, s: str) -> str:
        self.i = 0

        def helper():
            res = ""
            k = 0

            while self.i < len(s):
                c = s[self.i]

                if c.isdigit():
                    k = k * 10 + int(c)
                elif c == "[":
                    self.i += 1
                    res += k * helper()
                    k = 0
                elif c == "]":
                    return res
                else:
                    res += c

                self.i += 1
            return res

        return helper()

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

        for i in range(len(s)):
            if s[i] != "]":
                stack.append(s[i])
            else:
                substr = ""
                while stack[-1] != "[":
                    substr = stack.pop() + substr
                stack.pop()

                k = ""
                while stack and stack[-1].isdigit():
                    k = stack.pop() + k
                stack.append(int(k) * substr)

        return "".join(stack)

class Solution:
    def decodeString(self, s: str) -> str:
        string_stack = []
        count_stack = []
        cur = ""
        k = 0

        for c in s:
            if c.isdigit():
                k = k * 10 + int(c)
            elif c == "[":
                string_stack.append(cur)
                count_stack.append(k)
                cur = ""
                k = 0
            elif c == "]":
                temp = cur
                cur = string_stack.pop()
                count = count_stack.pop()
                cur += temp * count
            else:
                cur += c

        return cur