#updated

class Solution {
    public String customSortString(String order, String s) {
        StringBuilder res=new StringBuilder();
        int[] arr=new int[26];
        
        for(int i=0; i<s.length(); i++){
            arr[s.charAt(i)-'a']++;
        }
        
        for(int i=0; i<order.length(); i++){
            while(arr[order.charAt(i)-'a']>0){
                res.append(order.charAt(i));
                arr[order.charAt(i)-'a']--;
            }
        }
        
        for(int i=0; i<26; i++){
            while(arr[i]>0){
                res.append((char) (i+'a'));
                arr[i]--;
            }
        }
        
        return res.toString();
    }
}

p.s.: better runtime than previous one)