public String decodeString(String s) {

Stack<String> stack = new Stack<>();

for (int i = 0; i < s.length(); i++) {

if (s.charAt(i) == ']') {

StringBuilder subStr = new StringBuilder();

while (!stack.isEmpty() && !stack.peek().equals("[")) subStr.insert(0, stack.pop());

if (!stack.isEmpty()) {

stack.pop();

StringBuilder numStr = new StringBuilder();

while (!stack.isEmpty() && stack.peek().length() == 1 && Character.isDigit(stack.peek().charAt(0))) numStr.insert(0, stack.pop());

int num = Integer.parseInt(numStr.toString());

for (int j = 0; j < num; j++) stack.add(subStr.toString());

}

} else {

stack.add(Character.toString(s.charAt(i)));

}

}

StringBuilder res = new StringBuilder();

while (!stack.isEmpty()) {

res.insert(0, stack.pop());

}

return res.toString();

}