26. 394. Decode String

class Solution {
    public String decodeString(String s) { 
        Stack<String>st1 = new Stack<>();
        Stack<Integer>st2 = new Stack<>();
        String res = "";
        int num=0;
        for(int i =0;i <s.length();i++){
            if(s.charAt(i)>='0'&&s.charAt(i)<='9'){
                num = num*10+(s.charAt(i)-'0');
            }else if(s.charAt(i)=='['){
                st2.push(num);
                num=0;
                st1.push("[");
            }else if(s.charAt(i)==']'){
                int count = st2.pop();
                StringBuilder sb = new StringBuilder();
                while(!st1.isEmpty()&& st1.peek()!="["){
                    sb.insert(0,st1.pop());
                }
                st1.pop();
                StringBuilder temp = new StringBuilder();
                while(count-->0){
                    temp = temp.append(sb);
                }
               st1.push(temp.toString());

            }else{
                st1.push(s.charAt(i)+"");
            }
        }

              StringBuilder sb = new StringBuilder();
        while(!st1.isEmpty()){
                sb.insert(0,st1.pop());
        }
        res = res+ sb.toString();

        return res;
    }

    
}