public class Solution {

public String decodeString(String s) {

if (s == null)

return "";

int sLen = s.length();

int idx = 0;

StringBuilder sb = new StringBuilder();

while (idx < sLen) {

int idxOfFirstLetter = idx;

while (idx < sLen && !(s.charAt(idx) >= '0' && s.charAt(idx) <= '9')) {

idx++;

}

sb.append(s.substring(idxOfFirstLetter, idx));

if (idx < sLen) {

int idxOfFirstDigit = idx;

while (idx < sLen && s.charAt(idx) != '[') {

idx++;

}

int factor = Integer.valueOf(s.substring(idxOfFirstDigit, idx));

idx++;

int startIdx = idx;

int cnt = 0;

while (idx < sLen && (s.charAt(idx) != ']' || cnt > 0)) {

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

cnt++;

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

cnt--;

idx++;

}

String subStr = decodeString(s.substring(startIdx, idx));

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

sb.append(subStr);

}

idx++;

}

}

return sb.toString();

}

}