Longest Arithmetic Subsequence of Given Difference LeetCode solution in C++

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class Solution {
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 public:
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  int longestSubsequence(vector<int> &arr, int difference) {
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    int n = arr.size();
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    unordered_map<int, int> mp;
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    multiset<int> s;
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    int ans = 1;
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    for (int i = 0; i < n; i++) {
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      int req = arr[i] - difference;
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      auto it = s.find(req);
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      s.insert(arr[i]);
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      if (it == s.end()) {
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        continue;
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      } else {
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        mp[arr[i]] = mp[req] + 1;
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        ans = max(ans, mp[arr[i]] + 1);
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      }
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    }
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    return ans;
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  }
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};