https://leetcode.com/problems/number-of-connected-components-in-an-undirected-graph/description/

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这道题就是根据边连起来，看有几个集合。典型的并查集应用。

int[] parent;
    int find(int i){
        if(parent[i] == i) return i;
        return parent[i] = find(parent[i]);
    }
    boolean union(int i,int j){
        int pi = find(i);
        int pj = find(j);
        if(pi == pj) return false;
        parent[pi] = pj;
        return true;
    }
    public int countComponents(int n, int[][] edges) {
        parent = new int[n];
        for(int i=0;i<n;i++)
            parent[i] = i;
        int cnt = n;
        for(int[] edge : edges){
            if(union(edge[0],edge[1])) cnt--;
        }
        return cnt;
        
    }