Number of Connected Components in an Undirected Graph -- LeetCode 323

[Question]

Given n nodes labeled from 0 to n - 1 and a list of undirected edges (each edge is a pair of nodes), write a function to find the number of connected components in an undirected graph.

Example 1:

     0          3
     |          |
     1 --- 2    4

Given n = 5 and edges = [[0, 1], [1, 2], [3, 4]], return 2.

Example 2:

     0           4
     |           |
     1 --- 2 --- 3

Given n = 5 and edges = [[0, 1], [1, 2], [2, 3], [3, 4]], return 1.

Note:
You can assume that no duplicate edges will appear in edges. Since all edges are undirected, [0, 1] is the same as [1, 0] and thus will not appear together in edges.

[Analysis]
Classic Union-find problem.

[Solution]
class UnionFind {
    vector<int> u;

public:
    UnionFind(int n) {
        u =vector<int>(n,0);
        for (int i=0; i<n; i++)
            u[i] = i;
    }

    int find(int i) {
        if (u[i]==i) return i;
        u[i] = find(u[i]);
        return u[i];
    }

    void unions(int i, int j) {
        u[find(i)] = find(j);
    }
};

int countComponents(vector<pair<int,int>>& edges, int n) {
    UnionFind uf(n);
   
    for (auto& e:edges)
        uf.unions(e.first, e.second);
   
    unordered_set<int> res;
    for(int i=0; i<n; i++)
        res.insert(uf.find(i));

    return res.size();
}