public String solution(String s){

String answer = "";

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

for (char x : s.toCharArray()) {

if(x == ')') {

String tmp = "";

while(!st.isEmpty()) {

String c = st.pop();

if(c.equals("(")) {

String num = "";

while(!st.isEmpty() && Character.isDigit(st.peek().charAt(0))) {

num = st.pop() + num;

}

String res = "";

int cnt = 0;

if(num.equals("")) cnt = 1;

else cnt = Integer.parseInt(num);

for(int i = 0; i < cnt; i++) res += tmp;

st.push(res);

break;

}

tmp = c + tmp;

}

}

else st.push(String.valueOf(x));

}

for (String s1 : st) {

answer += s1;

}

return answer;

}

public static void main(String[] args){

Q3_2 T = new Q3_2();

System.out.println(T.solution("3(a2(b))ef"));

System.out.println(T.solution("2(ab)k3(bc)"));

System.out.println(T.solution("2(ab3((cd)))"));

System.out.println(T.solution("2(2(ab)3(2(ac)))"));

System.out.println(T.solution("3(ab2(sg))"));

}