Solution.java

class Solution {
    public String decodeString(String s) {
        Deque<Integer> s1 = new ArrayDeque<>();
        Deque<String> s2 = new ArrayDeque<>();
        int num = 0;
        String res = "";
        for (char c : s.toCharArray()) {
            if ('0' <= c && c <= '9') {
                num = num * 10 + c - '0';
            } else if (c == '[') {
                s1.push(num);
                s2.push(res);
                num = 0;
                res = "";
            } else if (c == ']') {
                StringBuilder t = new StringBuilder();
                for (int i = 0, n = s1.pop(); i < n; ++i) {
                    t.append(res);
                }
                res = s2.pop() + t.toString();
            } else {
                res += String.valueOf(c);
            }
        }
        return res;
    }
}