public String decodeString(String s) {

StringBuilder sb = new StringBuilder("");

int i = 0;

StringBuilder num = new StringBuilder("");

while (i < s.length()) {

char c = s.charAt(i);

if (c >= '0' && c <= '9') {

num.append(c);

i++;

continue;

}

if (c == '[') {

Res res = decodeString(s, i);

int next = res.next;

String in = res.in;

for (int j = 0; j<Integer.parseInt(num.toString()); j++) {

sb.append(in);

}

num = new StringBuilder("");

i = next;

continue;

}

if ((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z')) {

sb.append(c);

i++;

continue;

}

}

return sb.toString();

}

public Res decodeString(String s, int start) {

StringBuilder sb = new StringBuilder("");

int i = start+1;

StringBuilder num = new StringBuilder("");

while (i < s.length()) {

char c = s.charAt(i);

if (c >= '0' && c <= '9') {

num.append(c);

i++;

continue;

}

if (c == '[') {

Res res = decodeString(s, i);

int next = res.next;

String in = res.in;

for (int j = 0; j<Integer.parseInt(num.toString()); j++) {

sb.append(in);

}

num = new StringBuilder("");

i = next;

continue;

}

if (c == ']') {

return new Res(i+1, sb.toString());

}

if ((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z')) {

sb.append(c);

i++;

continue;

}

}

return new Res(i+1, sb.toString());

}

class Res {

int next;

String in;

Res (int next, String in) {

this.next = next;

this.in = in;

}

}

/**

public String decodeString2(String s) {

if (s.length() == 0) return "";

StringBuilder sb = new StringBuilder();

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

char c = s.charAt(i);

if (Character.isDigit(c)) {

int digit_begin = i;

while (s.charAt(i) != '[') i++;

int num = Integer.valueOf(s.substring(digit_begin, i));

int count = 1;

int str_begin = i+1;

i ++;

while (count != 0) {

if (s.charAt(i) == '[') count ++;

else if (s.charAt(i) == ']') count --;

i ++;

}

i--;

String str = decodeString(s.substring(str_begin, i));

for (int j = 0; j < num; j ++) {

sb.append(str);

}

} else {

sb.append(c);

}

}

return sb.toString();

}

/**

public String decodeString3(String s) {

String res = "";

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

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

int idx = 0;

while (idx < s.length()) {

if (Character.isDigit(s.charAt(idx))) {

int count = 0;

while (Character.isDigit(s.charAt(idx))) {

count = 10 * count + (s.charAt(idx) - '0');

idx++;

}

countStack.push(count);

}

else if (s.charAt(idx) == '[') {

resStack.push(res);

res = "";

idx++;

}

else if (s.charAt(idx) == ']') {

StringBuilder temp = new StringBuilder (resStack.pop());

int repeatTimes = countStack.pop();

for (int i = 0; i < repeatTimes; i++) {

temp.append(res);

}

res = temp.toString();

idx++;

}

else {

res += s.charAt(idx++);

}

}

return res;

}