decode-string.py

class Solution:
    def decodeString(self, s: str) -> str:
        multipliers = []
        sequences = []
        
        last_multiplier = 0
        last_sequence = ''
        
        for c in s:
            if c.isdigit():
                last_multiplier = last_multiplier * 10 + int(c)
            elif last_multiplier:
                multipliers.append(last_multiplier)
                last_multiplier = 0
                
            if c == '[':
                sequences.append(last_sequence)
                last_sequence = ''
            elif c == ']':
                prev_sequence = ''
                if sequences:
                    prev_sequence = sequences.pop()
                multiplier = multipliers.pop()
                last_sequence = prev_sequence + multiplier * last_sequence
            elif c.isalpha():
                last_sequence += c
        
        return last_sequence