Decode the Message.java

// Runtime: 12 ms (Top 48.65%) | Memory: 44.7 MB (Top 35.73%)
class Solution {
    public String decodeMessage(String key, String message) {
        StringBuilder ans = new StringBuilder();//Using String Builder to append the string
        key = key.replaceAll(" ", "");
        //Removing the spaces
        HashMap<Character,Character> letters = new HashMap<>();
        //Mapping the key into a hashmap.
        char original = 'a';
        for (int i = 0; i < key.length() ; i++) {
            if (!letters.containsKey(key.charAt(i))){
                letters.put(key.charAt(i),original++);
            }
        }
        //After the first pass all the letters of the key will be mapped with their respective original letters.
        for (int i = 0; i < message.length(); i++) {
            if (letters.containsKey(message.charAt(i))){
                //Now replacing the letters of the message with appropriate letter according to the key
                ans.append(letters.get(message.charAt(i)));
            }else{
                ans.append(message.charAt(i));
                //This is for characters other than the letters in the key example a space " "
                //They will not be replaced by any letters hence original letter is appended into the StringBuilder
            }
        }
        return ans.toString();
    }
}