Decode String — Stack for Counts and Strings

# Python
def decodeString(s):
    stack = []  # (current_string, current_count)
    cur_str = ''
    cur_num = 0
    for c in s:
        if c.isdigit():
            cur_num = cur_num * 10 + int(c)
        elif c == '[':
            stack.append((cur_str, cur_num))
            cur_str = ''; cur_num = 0
        elif c == ']':
            prev_str, num = stack.pop()
            cur_str = prev_str + num * cur_str
        else:
            cur_str += c
    return cur_str

// Java
public String decodeString(String s) {
    Deque<Integer> counts = new ArrayDeque<>();
    Deque<StringBuilder> strings = new ArrayDeque<>();
    StringBuilder cur = new StringBuilder(); int k = 0;
    for (char c : s.toCharArray()) {
        if (Character.isDigit(c)) k = k*10 + c-'0';
        else if (c == '[') { counts.push(k); strings.push(cur); cur = new StringBuilder(); k=0; }
        else if (c == ']') {
            int n = counts.pop(); StringBuilder prev = strings.pop();
            for (int i=0;i<n;i++) prev.append(cur); cur = prev;
        } else cur.append(c);
    }
    return cur.toString();
}

// C++
string decodeString(string s) {
    stack<int> counts; stack<string> strings;
    string cur; int k = 0;
    for (char c : s) {
        if (isdigit(c)) k=k*10+c-'0';
        else if (c=='[') { counts.push(k); strings.push(cur); cur=""; k=0; }
        else if (c==']') {
            int n=counts.top(); counts.pop();
            string prev=strings.top(); strings.pop();
            for (int i=0;i<n;i++) prev+=cur; cur=prev;
        } else cur+=c;
    }
    return cur;
}