DisjointSet

package com.zhuyu_deng.test;

//DisjSets class
//
//CONSTRUCTION: with int representing initial number of sets
//
//******************PUBLIC OPERATIONS*********************
//void union( root1, root2 ) --> Merge two sets
//int find( x )              --> Return set containing x
//******************ERRORS********************************
//No error checking is performed

/**
 * Disjoint set class, using union by rank and path compression. Elements in the
 * set are numbered starting at 0.
 * 
 * @author Mark Allen Weiss
 */
public class DisjSets
{
	// 定义数组变量
	private int[] s;
	/**
	 * Construct the disjoint sets object.
	 * 
	 * @param numElements
	 *            the initial number of disjoint sets.
	 */
	public DisjSets(int numElements)
	{
		s = new int[numElements];
		for (int i = 0; i < s.length; i++)
			s[i] = -1;
	}

	/**
	 * Union two disjoint sets using the height heuristic. For simplicity, we
	 * assume root1 and root2 are distinct and represent set names.
	 * 
	 * @param root1
	 *            the root of set 1.
	 * @param root2
	 *            the root of set 2.
	 */
	public void union(int root1, int root2)  // 小树接到大树上
	{
		if (s[root2] < s[root1]) // root2 is deeper
			s[root1] = root2; // Make root2 new root
		else
		{
			if (s[root1] == s[root2])
				s[root1]--; // Update height if same
			s[root2] = root1; // Make root1 new root
		}
	}

	/**
	 * Perform a find with path compression. Error checks omitted again for
	 * simplicity.
	 * 
	 * @param x
	 *            the element being searched for.
	 * @return the set containing x.
	 */
	// 寻找x所在集合的代表元
	public int find(int x)
	{
		if (s[x] < 0)
			return x;
		else
			return s[x] = find(s[x]);  // 路径压缩
	}

	

	// Test main; all finds on same output line should be identical
	public static void main(String[] args)
	{
		int NumElements = 128;
		int NumInSameSet = 16;

		DisjSets ds = new DisjSets(NumElements);
		int set1, set2;

		for (int k = 1; k < NumInSameSet; k *= 2)
		{
			for (int j = 0; j + k < NumElements; j += 2 * k)
			{
				set1 = ds.find(j);
				set2 = ds.find(j + k);
				ds.union(set1, set2);
			}
		}

		for (int i = 0; i < NumElements; i++)
		{
			System.out.print(ds.find(i) + "*");
			if (i % NumInSameSet == NumInSameSet - 1)
				System.out.println();
		}
		System.out.println();
	}
}