Design a Number Container System LeetCode solution in C++

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class NumberContainers {
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 public:
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  map<int, int> indexToNumber;      // stores number corresponding to an index.
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  map<int, set<int>> numberToIndex; // stores all the indexes corresponding to a number.
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  NumberContainers() {}
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  void change(int index, int number) {
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    if (!indexToNumber.count(index)) {     // if there is no number at the given index.
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      numberToIndex[number].insert(index); // store index corresponding to the given number
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      indexToNumber[index] = number;       // store number corresponding to the index.
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    } else {                               // Update both map.
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      int num = indexToNumber[index]; // number at given index currently.
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      // remove the index
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      numberToIndex[num].erase(index);
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      if (numberToIndex[num].empty()) numberToIndex.erase(num);
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      // insert the new number at the given index and store the index
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      // corresponding to that number.
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      numberToIndex[number].insert(index);
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      indexToNumber[index] = number;
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    }
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  }
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  int find(int number) {
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    if (!numberToIndex.count(number)) return -1;
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    // returning first element in the set as it will be the smallest index
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    // always.
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    return *numberToIndex[number].begin();
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  }
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};