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Merged
merged 1 commit into from
Mar 15, 2019
Merged

I added an implementation of disjoint-set #716

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136 changes: 136 additions & 0 deletions src/main/java/com/dataStructures/DisjointSet.java
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package src.main.java.com.dataStructures;

import java.io.Serializable;
import java.util.*;

/**
* An implementation of disjoint-set, represented by rooted trees.
* <p>
* Actually, the instance of {@link DisjointSet} is a disjoint-set forests.
*
* <p>
* Disjoint-set operations:
* <p>
* 1. quickly unites two sets into a new set, requiring O(1) time.
* <p>
* 2. quickly query two elements whether contained in the same set, requiring about O(1) time.
*
* @author yangxf
*/
public class DisjointSet<T> implements Serializable {
private static final long serialVersionUID = 3134700471905625636L;

private Map<T, Node<T>> nodeMap = new HashMap<>();

/**
* Add an element to the disjoint-set forests as a set.
*/
public void makeSet(T element) {
checkNotNull(element, "element");
nodeMap.putIfAbsent(element, new Node<>());
}

/**
* Unites the set that contains left and the set that contains right
* into a new set with the union-by-rank heuristic.
* <p>
* Rank is an upper bound on the height of node.
*/
public void union(T left, T right) {
checkNotNull(left, "element");
checkNotNull(right, "element");

Node<T> leftNode = nodeMap.get(left),
rightNode = nodeMap.get(right);

if (leftNode == null) {
throw new NoSuchElementException(left.toString());
}

if (rightNode == null) {
throw new NoSuchElementException(right.toString());
}

Node<T> leftSet = findSet(leftNode),
rightSet = findSet(rightNode);

if (leftSet == rightSet) {
return;
}

if (leftSet.rank < rightSet.rank) {
leftSet.parent = rightSet;
} else {
rightSet.parent = leftSet;
if (leftSet.rank == rightSet.rank) {
leftSet.rank++;
}
}
}

/**
* Query two elements whether contained in the same set.
*/
public boolean isConnected(T left, T right) {
if (left == null || right == null) {
return false;
}

Node<T> leftNode = nodeMap.get(left);
if (leftNode == null) {
return false;
}

Node<T> rightNode = nodeMap.get(right);
if (rightNode == null) {
return false;
}

if (leftNode == rightNode) {
return true;
}

return findSet(leftNode) == findSet(rightNode);
}

public Collection<Set<T>> toSets() {
Map<Node<T>, Set<T>> setMap = new HashMap<>();
for (Map.Entry<T, Node<T>> entry : nodeMap.entrySet()) {
setMap.computeIfAbsent(findSet(entry.getValue()), k -> new HashSet<>())
.add(entry.getKey());
}
return setMap.values();
}

public void show() {
toSets().forEach(System.out::println);
}

/**
* Find the set that contains the node, actual is the first node of the set.
* <p>
* Backtracking with path compression.
*/
private Node<T> findSet(Node<T> node) {
if (node != node.parent) {
node.parent = findSet(node.parent);
}
return node.parent;
}

private static void checkNotNull(Object obj, String msg) {
if (obj == null) {
throw new NullPointerException(msg + " must be not null");
}
}

static class Node<T> {
int rank;
Node<T> parent;

Node() {
parent = this;
}
}

}
34 changes: 34 additions & 0 deletions src/test/java/com/dataStructures/DisjointSetTest.java
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package src.test.java.com.dataStructures;

import org.junit.Test;
import src.main.java.com.dataStructures.DisjointSet;

import static org.junit.Assert.*;

public class DisjointSetTest {
@Test
public void test() {
DisjointSet<Object> set = new DisjointSet<>();

set.makeSet("flink");
set.makeSet("c++");
set.makeSet("java");
set.makeSet("py");
set.makeSet("spark");

set.union("java", "c++");

assertTrue(set.isConnected("java", "c++"));
assertFalse(set.isConnected("java", "py"));

set.union("c++", "py");
assertTrue(set.isConnected("java", "py"));

set.makeSet("lisp");
set.union("lisp", "py");

assertTrue(set.isConnected("c++", "lisp"));

set.show();
}
}