decodeString.java

class Solution {
    public String decodeString(String s) {
        Stack<String> stack= new Stack<>();

        for(int i=0;i<s.length();i++){
            if(s.charAt(i)!=']'){
                stack.push(String.valueOf(s.charAt(i)));
            }
            else{
                StringBuilder substr=new StringBuilder();
                while(!stack.peek().equals("[")){
                    substr.insert(0,stack.pop());
                }
                stack.pop();

                StringBuilder k=new StringBuilder();

                while(!stack.isEmpty() && Character.isDigit(stack.peek().charAt(0))){
                    k.insert(0,stack.pop());
                }

                int count=Integer.parseInt(k.toString());
                String repeatedstr=substr.toString().repeat(count);
                stack.push(repeatedstr);
            }
        }
        StringBuilder res= new StringBuilder();
        while(!stack.isEmpty()){
            res.insert(0,stack.pop());
        }
        return res.toString();
    }
}