Decode String

You are given an encoded string where k[encoded_string] means the encoded_string inside the brackets is repeated exactly k times. You may assume the input string is always valid.

Example:

• Input: 3[a]2[bc] -> Output: aaabcbc
• Input: 3[a2[c]] -> Output: accaccacc (Nested!)

We use a Stack to handle the nested nature of the problem.

1. Initialize an empty stack.
2. Iterate through the string character by character.
3. If the char is NOT ], push it to the stack.
4. If the char IS ], we must start decoding:
   • Pop characters until we find [. This is the string_to_repeat.
   • Pop the [.
   • Pop numbers to form k (the repetition count).
   • Repeat string_to_repeat k times and Push it back to the stack.
5. Finally, join the stack to get the result.