Check Array Formation Through Concatenation LeetCode solution in C++

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class Solution {
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 public:
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  bool canFormArray(vector<int> &arr, vector<vector<int>> &pieces) {
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    map<int, vector<int>> mp;
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    // map the 1st element in pieces[i] to pieces[i]
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    for (auto p : pieces) mp[p[0]] = p;
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    vector<int> result;
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    for (auto a : arr) {
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      if (mp.find(a) != mp.end()) result.insert(result.end(), mp[a].begin(), mp[a].end());
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    }
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    return result == arr;
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  }
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};