394. Decode String

class Solution {
    public String decodeString(String s) {
        Deque<Integer> numStack = new ArrayDeque<>();
        Deque<String> strStack = new ArrayDeque<>();
        StringBuilder tail = new StringBuilder();

        int n = s.length();
        for (int i = 0; i < n; i++) {
            char c = s.charAt(i);

            if (Character.isDigit(c)) {
                int num = c - '0';
                while (i + 1 < n && Character.isDigit(s.charAt(i + 1))) {
                    num = num * 10 + s.charAt(i + 1) - '0';
                    i++;
                }

                numStack.push(num);
            } else if (c == '[') {
                strStack.push(tail.toString());
                tail = new StringBuilder();
            } else if (c == ']') {
                StringBuilder tmp = new StringBuilder(strStack.pop());

                int repeatedTimes = numStack.pop();
                for (int j = 0; j < repeatedTimes; j++) {
                    tmp.append(tail);
                }

                tail = tmp;
            } else {
                tail.append(c);
            }
        }

        return tail.toString();
    }
}