Design a Number Container System LeetCode solution in Java

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class NumberContainers {
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  Map<Integer, TreeSet<Integer>> map;
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  Map<Integer, Integer> m;
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  public NumberContainers() {
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    map = new HashMap<>();
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    m = new HashMap<>();
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  }
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  public void change(int index, int number) {
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    m.put(index, number);
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    if (!map.containsKey(number)) map.put(number, new TreeSet<>());
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    map.get(number).add(index);
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  }
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  public int find(int number) {
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    if (!map.containsKey(number)) return -1;
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    for (Integer a : map.get(number)) {
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      if (m.get(a) == number) return a;
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    }
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    return -1;
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  }
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}