static String decoding(String s) {
		int count = 0;
		StringBuilder ret = new StringBuilder();
		for (char c : s.toCharArray()) {
			if (Character.isDigit(c)) {
				count = count * 10 + c - '0';
			} else { // isalpha.
				for (int i = 1; i <= count; i++) {
					ret.append(c);
				}
				count = 0;
			}
		}
		return ret.toString();
	}