Decode String

def decodeString(s):
    # Expand one innermost group at a time until no bracket is left.
    while True:
        close = s.find("]")
        if close == -1:
            return s
        # The first ']' closes a group with no group inside it,
        # and the nearest '[' to its left opens that group.
        open_ = s.rfind("[", 0, close)
        start = open_
        while start > 0 and s[start - 1].isdigit():
            start -= 1
        times = int(s[start:open_])
        s = s[:start] + s[open_ + 1:close] * times + s[close + 1:]

def decodeString(s):
    pos = 0

    def decode():
        # Read from pos up to the ']' that closes this level, or the end.
        nonlocal pos
        parts = []
        while pos < len(s) and s[pos] != "]":
            if s[pos].isdigit():
                times = 0
                while s[pos].isdigit():
                    times = times * 10 + int(s[pos])
                    pos += 1
                pos += 1          # skip '['
                inner = decode()  # the group's body, fully decoded
                pos += 1          # skip ']'
                parts.append(inner * times)
            else:
                parts.append(s[pos])
                pos += 1
        return "".join(parts)

    return decode()

def decodeString(s):
    stack = []      # one entry per open '[': (the text before it, its count)
    current = []    # pieces of the text at the current level
    count = 0
    for ch in s:
        if ch.isdigit():
            count = count * 10 + int(ch)   # counts can have several digits
        elif ch == "[":
            stack.append((current, count))
            current, count = [], 0
        elif ch == "]":
            before, times = stack.pop()
            before.append("".join(current) * times)
            current = before
        else:
            current.append(ch)
    return "".join(current)