import java.util.Stack;

public class DecodeString {

public String decodeString(String s) {

if(s == null || s.length() == 0) return s;

char[] ch = s.toCharArray();

Stack<Integer> numStack = new Stack<>();

Stack<Character> charStack = new Stack<>();

int k = 0;

boolean isDigit = false;

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

if(Character.isDigit(ch[i])) {

k = k * 10 + (ch[i] - '0');

isDigit = true;

continue;

}

if(isDigit){

numStack.push(k);

k = 0;

isDigit = false;

}

if(ch[i] != ']'){

charStack.push(ch[i]);

continue;

}

if(ch[i] == ']'){

StringBuilder sb = new StringBuilder();

while(!charStack.isEmpty() && charStack.peek() != '['){

sb.append(charStack.pop());

}

charStack.pop();

int time = numStack.pop();

String temp = sb.reverse().toString();

while(time-- > 0){

for(int j = 0; j < temp.length(); j++)

charStack.push(temp.charAt(j));

}

}

}

StringBuilder sb = new StringBuilder();

while(!charStack.isEmpty()){

sb.append(charStack.pop());

}

return sb.reverse().toString();

}

public static void main(String[] args) {

DecodeString decode = new DecodeString();

System.out.println(decode.decodeString("3[a2[c]]"));

System.out.println(decode.decodeString("2[abc]3[cd]ef"));

System.out.println(decode.decodeString("10[2[abc]3[cd]ef]"));

System.out.println(decode.decodeString("asdes"));

}

}