public class UF {
    private int[] id;
    private int count;
    public UF(int N){
       // 初始化分量Id陣列
        count=N;
        id= new int[N];
        for (int i = 0; i <N ; i++) {
            id[i] = i;
        }
    }
    public int count(){return count;}
    public boolean connected(int p,int q){
        return find(p)==find(q);
    }
    public int find(int p){return id[p];}
    public void union(int p ,int q){
        //將pq歸併到相同的分量中
        int pId=find(p);
        int qId=find(q);
        //如果P和q已經在相同的分量之中則不需要採取任何行動
        if(pId==qId) return;
        //將P的分量重新命名為q的名稱
        for (int i = 0; i <id.length ; i++) {
            if (id[i]==pId) id[i]=qId;
        }
        count--;
    }

    public static void main(String[] args) {
        //解決有in得到的動態連通性問題
        Scanner scanner = new Scanner(System.in);
        int N = Integer.parseInt(scanner.nextLine());
        UF uf = new UF(N);
        System.out.println("請告訴輸出多少個數據");
        int i=Integer.parseInt(scanner.nextLine());
        int j=0;
        while (j<i){
            int p= scanner.nextInt();
            int q= scanner.nextInt();
            j++;
            if (uf.connected(p,q)) {
                continue;
            }//如果已經連通就忽略
            uf.union(p,q);
            System.out.println(p+" " +q);

        }
        System.out.println(uf.count() + 0+" components");
    }

    private int find2(int p){
         //找出分量名稱
        while (p!=id[p]) p=id[p];
        return p;
    }


    //對union的改進0
    public void union2(int p,int q){
        //將P和q的根節點統一
        int pRoot = find(p);
        int qRoot = find(q);
        if (pRoot == qRoot) return;

        id[pRoot] =qRoot;
        count--;
    }


}
 

優化版

public class WeightedQuickUnionUF {
    private int[] parent;   // parent[i] = parent of i  父連結陣列
    private int[] size;     // size[i] = number of sites in subtree rooted at i 各個節點所對應的分量大小
    private int count;      // number of components  連通分量的數量
    public WeightedQuickUnionUF(int n) {
        count = n;
        parent = new int[n];
        size = new int[n];
        for (int i = 0; i < n; i++) {
            parent[i] = i;
            size[i] = 1;
        }
    }

    public int count() {
        return count;
    }
    public int find(int p) {
        validate(p);
        while (p != parent[p])
            p = parent[p];
        return p;
    }
    // validate that p is a valid index
    private void validate(int p) {
        int n = parent.length;
        if (p < 0 || p >= n) {
            throw new IllegalArgumentException("index " + p + " is not between 0 and " + (n-1));
        }
    }


    public boolean connected(int p, int q) {
        return find(p) == find(q);
    }

    public void union(int p, int q) {
        int rootP = find(p);
        int rootQ = find(q);
        if (rootP == rootQ) return;

        // make smaller root point to larger one
        if (size[rootP] < size[rootQ]) {
            parent[rootP] = rootQ;
            size[rootQ] += size[rootP];
        }
        else {
            parent[rootQ] = rootP;
            size[rootP] += size[rootQ];
        }
        count--;
    }



    public static void main(String[] args) {
        int n = StdIn.readInt();
        edu.princeton.cs.algs4.WeightedQuickUnionUF uf = new edu.princeton.cs.algs4.WeightedQuickUnionUF(n);
        while (!StdIn.isEmpty()) {
            int p = StdIn.readInt();
            int q = StdIn.readInt();
            if (uf.connected(p, q)) continue;
            uf.union(p, q);
            StdOut.println(p + " " + q);
        }
        StdOut.println(uf.count() + " components");
    }

}