Decode String.java

class Solution {
    public String decodeString(String s) {
        String res = "";
        Stack<Integer> countStack = new Stack<>();
        Stack<String> resStack = new Stack<>();
        
        int index = 0;
        
        while(index < s.length())
        {
            if(Character.isDigit(s.charAt(index)))
            {
                int count = 0;
                while(Character.isDigit(s.charAt(index)))
                {
                    count = 10 * count + (s.charAt(index) - '0');
                    index++;
                }
                countStack.push(count);
            }
            else if(s.charAt(index) == '[')
            {
                resStack.push(res);
                res = "";
                index++;
            }
            else if(s.charAt(index) == ']')
            {
                StringBuilder temp = new StringBuilder(resStack.pop());
                int x = countStack.pop();
                for(int i = 0; i<x; i++)
                    temp.append(res);
                res = temp.toString();
                index++;
            }
            else{
                res += s.charAt(index);
                index++;
            }
            
        }
        return res;
    }
}