Check If Array Pairs Are Divisible by k LeetCode solution in C++

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// ask the interviewer
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// can we have negative values?
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class Solution {
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 public:
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  bool canArrange(vector<int> &arr, int k) {
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    unordered_map<int, int> memo; // remainder : occurence
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    // we are storing the frequency of curr%k
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    // i.e. the frequency of the remainder
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    for (auto curr : arr) {
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      int remainder = ((curr % k) + k) % k;
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      memo[remainder]++;
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    }
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    for (int i = 1; i <= k / 2; i++) {
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      if (memo[i] != memo[k - i]) return false;
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    }
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    if (memo[0] % 2 != 0) return false;
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    return true;
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  }
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};