Find Center of Star Graph LeetCode solution in C++

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class Solution {
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 public:
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  int findCenter(vector<vector<int>> &edges) {
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    // create an adjacency list
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    unordered_map<int, set<int>> adj;
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    int maxi;
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    for (int i = 0; i < edges.size(); i++) {
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      int u = edges[i][0];
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      int v = edges[i][1];
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      maxi = max(maxi, max(u, v));
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      adj[u].insert(v);
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      adj[v].insert(u);
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    }
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    for (auto i : adj) {
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      if (i.second.size() == maxi - 1) return i.first;
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    }
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    return -1;
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  }
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};