We process characters one by one. When we see a digit, we parse the full number k. When we see [, we recurse to decode the inner content. When we see ], we return from the current recursion.

1. Use an index pointer i that is shared or passed across recursion.
2. solve():
   • Initialize res = "".
   • While i < len(s) and s[i] != ']':
     • If s[i] is not a digit: res += s[i], i += 1.
     • If s[i] is a digit:
       • Parse k.
       • i += 1 (skip [).
       • inner_str = solve().
       • i += 1 (skip ]).
       • res += inner_str * k.
   • Return res.

• Time Complexity: O(N) where N is the length of decoded string.
• Space Complexity: O(N).

def decode_string(s):
    def solve(i):
        res = ""
        while i < len(s) and s[i] != ']':
            if not s[i].isdigit():
                res += s[i]
                i += 1
            else:
                k = 0
                while i < len(s) and s[i].isdigit():
                    k = k * 10 + int(s[i])
                    i += 1
                i += 1 # skip '['
                inner, next_i = solve(i)
                i = next_i + 1 # skip ']'
                res += inner * k
        return res, i
        
    return solve(0)[0]

(Alternative: Pass a list [0] as index to make it mutable).