Decode the string

import java.util.*;

class Solution {
    static String decodeString(String s) {
        Stack<Integer> countStack = new Stack<>();
        Stack<String> stringStack = new Stack<>();
        String currentString = "";
        int k = 0;

        for (char ch : s.toCharArray()) {
            if (Character.isDigit(ch))
                k = k * 10 + (ch - '0');
            else if (ch == '[') {
                countStack.push(k);
                stringStack.push(currentString);
                currentString = "";
                k = 0;
            } else if (ch == ']') {
                StringBuilder decodedString = new StringBuilder(stringStack.pop());
                int repeatTimes = countStack.pop();
                for (int i = 0; i < repeatTimes; i++)
                    decodedString.append(currentString);

                currentString = decodedString.toString();
            } else
                currentString += ch;
        }

        return currentString;
    }
}