LeetCode 1579. Remove Max Number of Edges to Keep Graph Fully Traversable

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class UF {
public:
    vector<int> f;
    vector<int> size;
    int n;
    int setCount; // count of current connected components
    UF(int _n): n(_n), setCount(_n), f(_n), size(_n, 1) {
        iota(f.begin(), f.end(), 0);
    }
    int find(int x) {
        return f[x] == x ? x : f[x] = find(f[x]);
    }
    bool _union(int x, int y) {
        x = find(x);
        y = find(y);
        if (x == y)
            return false;
        if (size[x] < size[y])
            swap(x, y);
        f[y] = x;
        size[x] += size[y];
        --setCount;
        return true;
    }
};
class Solution {
public:
    int maxNumEdgesToRemove(int n, vector<vector<int>>& edges) {
        UF ufa(n + 1), ufb(n + 1); // 0 as one dummy node
        int ans = 0;
        for (const auto& edge : edges)
            if (edge[0] == 3)
                if (!ufa._union(edge[1], edge[2]))
                    ++ans;
                else
                    ufb._union(edge[1], edge[2]);
        for (const auto& edge : edges)
            if (edge[0] == 1) {
                if (!ufa._union(edge[1], edge[2]))
                    ++ans;
            } else if (edge[0] == 2) {
                if (!ufb._union(edge[1], edge[2]))
                    ++ans;
            }
        if (ufa.setCount != 2 || ufb.setCount != 2) // 1 connected component and a dummy node
            return -1;
        return ans;
    }
};