content: /****************************************************************************
   *  Compilation:  javac QuickUnionUF.java
   *  Execution:  java QuickUnionUF < input.txt
   *  Dependencies: StdIn.java StdOut.java
   *
   *  Quick-union algorithm.
   *
   ****************************************************************************/
  
  public class QuickUnionUF {
      private int[] id;    // id[i] = parent of i
      private int count;   // number of components
  
      // instantiate N isolated components 0 through N-1
      public QuickUnionUF(int N) {
          id = new int[N];
          count = N;
          for (int i = 0; i < N; i++) {
              id[i] = i;
          }
      }
  
      // return number of connected components
      public int count() {
          return count;
      }
  
      // return root of component corresponding to element p
      public int find(int p) {
          while (p != id[p])
              p = id[p];
          return p;
      }
  
      // are elements p and q in the same component?
      public boolean connected(int p, int q) {
          return find(p) == find(q);
      }
  
      // merge components containing p and q
      public void union(int p, int q) {
          int i = find(p);
          int j = find(q);
          if (i == j) return;
          id[i] = j; 
          count--;
      }
  
      public static void main(String[] args) {
          int N = StdIn.readInt();
          QuickUnionUF uf = new QuickUnionUF(N);
  
          // read in a sequence of pairs of integers (each in the range 0 to N-1),
           // calling find() for each pair: If the members of the pair are not already
          // call union() and print the pair.
          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("# components: " + uf.count());
      }
  
  
  }
  