394.Decode String

1- Stack

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