Decode the Message LeetCode solution in Java

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