1class Solution {2    public String decodeString(String s) {3        Deque<Integer> counts = new ArrayDeque<>();4        Deque<StringBuilder> prefixes = new ArrayDeque<>();5        StringBuilder current = new StringBuilder();6        int number = 0;7        for (char c : s.toCharArray()) {8            if (Character.isDigit(c)) {9                number = number * 10 + c - '0';10            } else if (c == '[') {11                counts.push(number); prefixes.push(current);12                current = new StringBuilder(); number = 0;13            } else if (c == ']') {14                StringBuilder parent = prefixes.pop();15                int repeat = counts.pop();16                for (int i = 0; i < repeat; i++) parent.append(current);17                current = parent;18            } else current.append(c);19        }20        return current.toString();21    }22}