Iterator for Combination LeetCode solution in Java

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class CombinationIterator {
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  private Queue<String> allCombinations;
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  public CombinationIterator(String characters, int combinationLength) {
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    this.allCombinations = new LinkedList<>();
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    generateAllCombinations(characters, 0, combinationLength, new StringBuilder());
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  }
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  private void generateAllCombinations(
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      String characters, int index, int combinationLength, StringBuilder currentString) {
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    if (currentString.length() == combinationLength) {
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      this.allCombinations.offer(currentString.toString());
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      return;
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    }
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    for (int i = index; i < characters.length(); i++) {
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      currentString.append(characters.charAt(i));
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      generateAllCombinations(characters, i + 1, combinationLength, currentString);
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      currentString.deleteCharAt(currentString.length() - 1);
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    }
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  }
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  public String next() {
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    return this.allCombinations.poll();
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  }
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  public boolean hasNext() {
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    return !this.allCombinations.isEmpty();
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  }
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}