Add Maximum Weighted Matching Algorithm for Trees (#6184)

src/main/java/com/thealgorithms/datastructures/graphs/UndirectedAdjacencyListGraph.java

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+package com.thealgorithms.datastructures.graphs;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.HashSet;
+
+public class UndirectedAdjacencyListGraph {
+    private ArrayList<HashMap<Integer, Integer>> adjacencyList = new ArrayList<>();
+
+    /**
+     * Adds a new node to the graph by adding an empty HashMap for its neighbors.
+     * @return the index of the newly added node in the adjacency list
+     */
+    public int addNode() {
+        adjacencyList.add(new HashMap<>());
+        return adjacencyList.size() - 1;
+    }
+
+    /**
+     * Adds an undirected edge between the origin node (@orig) and the destination node (@dest) with the specified weight.
+     * If the edge already exists, no changes are made.
+     * @param orig the index of the origin node
+     * @param dest the index of the destination node
+     * @param weight the weight of the edge between @orig and @dest
+     * @return true if the edge was successfully added, false if the edge already exists or if any node index is invalid
+     */
+    public boolean addEdge(int orig, int dest, int weight) {
+        int numNodes = adjacencyList.size();
+        if (orig >= numNodes || dest >= numNodes || orig < 0 || dest < 0) {
+            return false;
+        }
+
+        if (adjacencyList.get(orig).containsKey(dest)) {
+            return false;
+        }
+
+        adjacencyList.get(orig).put(dest, weight);
+        adjacencyList.get(dest).put(orig, weight);
+        return true;
+    }
+
+    /**
+     * Returns the set of all adjacent nodes (neighbors) for the given node.
+     * @param node the index of the node whose neighbors are to be retrieved
+     * @return a HashSet containing the indices of all neighboring nodes
+     */
+    public HashSet<Integer> getNeighbors(int node) {
+        return new HashSet<>(adjacencyList.get(node).keySet());
+    }
+
+    /**
+     * Returns the weight of the edge between the origin node (@orig) and the destination node (@dest).
+     * If no edge exists, returns null.
+     * @param orig the index of the origin node
+     * @param dest the index of the destination node
+     * @return the weight of the edge between @orig and @dest, or null if no edge exists
+     */
+    public Integer getEdgeWeight(int orig, int dest) {
+        return adjacencyList.get(orig).getOrDefault(dest, null);
+    }
+
+    /**
+     * Returns the number of nodes currently in the graph.
+     * @return the number of nodes in the graph
+     */
+    public int size() {
+        return adjacencyList.size();
+    }
+}

src/main/java/com/thealgorithms/dynamicprogramming/TreeMatching.java

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+package com.thealgorithms.dynamicprogramming;
+
+import com.thealgorithms.datastructures.graphs.UndirectedAdjacencyListGraph;
+
+/**
+ * This class implements the algorithm for calculating the maximum weighted matching in a tree.
+ * The tree is represented as an undirected graph with weighted edges.
+ *
+ * Problem Description:
+ *  Given an undirected tree G = (V, E) with edge weights γ: E → N and a root r ∈ V,
+ *  the goal is to find a maximum weight matching M ⊆ E such that no two edges in M
+ *  share a common vertex. The sum of the weights of the edges in M, ∑ e∈M γ(e), should be maximized.
+ *  For more Information: <a href="https://en.wikipedia.org/wiki/Matching_(graph_theory)">Matching (graph theory)</a>
+ *
+ * @author  <a href="https://github.com/DenizAltunkapan">Deniz Altunkapan</a>
+ */
+public class TreeMatching {
+
+    private UndirectedAdjacencyListGraph graph;
+    private int[][] dp;
+
+    /**
+     * Constructor that initializes the graph and the DP table.
+     *
+     * @param graph The graph that represents the tree and is used for the matching algorithm.
+     */
+    public TreeMatching(UndirectedAdjacencyListGraph graph) {
+        this.graph = graph;
+        this.dp = new int[graph.size()][2];
+    }
+
+    /**
+     * Calculates the maximum weighted matching for the tree, starting from the given root node.
+     *
+     * @param root The index of the root node of the tree.
+     * @param parent The index of the parent node (used for recursion).
+     * @return The maximum weighted matching for the tree, starting from the root node.
+     *
+     */
+    public int getMaxMatching(int root, int parent) {
+        if (root < 0 || root >= graph.size()) {
+            throw new IllegalArgumentException("Invalid root: " + root);
+        }
+        maxMatching(root, parent);
+        return Math.max(dp[root][0], dp[root][1]);
+    }
+
+    /**
+     * Recursively computes the maximum weighted matching for a node, assuming that the node
+     * can either be included or excluded from the matching.
+     *
+     * @param node The index of the current node for which the matching is calculated.
+     * @param parent The index of the parent node (to avoid revisiting the parent node during recursion).
+     */
+    private void maxMatching(int node, int parent) {
+        dp[node][0] = 0;
+        dp[node][1] = 0;
+
+        int sumWithoutEdge = 0;
+        for (int adjNode : graph.getNeighbors(node)) {
+            if (adjNode == parent) {
+                continue;
+            }
+            maxMatching(adjNode, node);
+            sumWithoutEdge += Math.max(dp[adjNode][0], dp[adjNode][1]);
+        }
+
+        dp[node][0] = sumWithoutEdge;
+
+        for (int adjNode : graph.getNeighbors(node)) {
+            if (adjNode == parent) {
+                continue;
+            }
+            int weight = graph.getEdgeWeight(node, adjNode);
+            dp[node][1] = Math.max(dp[node][1], sumWithoutEdge - Math.max(dp[adjNode][0], dp[adjNode][1]) + dp[adjNode][0] + weight);
+        }
+    }
+}

src/test/java/com/thealgorithms/dynamicprogramming/TreeMatchingTest.java

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+package com.thealgorithms.dynamicprogramming;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import com.thealgorithms.datastructures.graphs.UndirectedAdjacencyListGraph;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+class TreeMatchingTest {
+    UndirectedAdjacencyListGraph graph;
+
+    @BeforeEach
+    void setUp() {
+        graph = new UndirectedAdjacencyListGraph();
+        for (int i = 0; i < 14; i++) {
+            graph.addNode();
+        }
+    }
+
+    @Test
+    void testMaxMatchingForGeneralTree() {
+        graph.addEdge(0, 1, 20);
+        graph.addEdge(0, 2, 30);
+        graph.addEdge(1, 3, 40);
+        graph.addEdge(1, 4, 10);
+        graph.addEdge(2, 5, 20);
+        graph.addEdge(3, 6, 30);
+        graph.addEdge(3, 7, 30);
+        graph.addEdge(5, 8, 40);
+        graph.addEdge(5, 9, 10);
+
+        TreeMatching treeMatching = new TreeMatching(graph);
+        assertEquals(110, treeMatching.getMaxMatching(0, -1));
+    }
+
+    @Test
+    void testMaxMatchingForBalancedTree() {
+        graph.addEdge(0, 1, 20);
+        graph.addEdge(0, 2, 30);
+        graph.addEdge(0, 3, 40);
+        graph.addEdge(1, 4, 10);
+        graph.addEdge(1, 5, 20);
+        graph.addEdge(2, 6, 20);
+        graph.addEdge(3, 7, 30);
+        graph.addEdge(5, 8, 10);
+        graph.addEdge(5, 9, 20);
+        graph.addEdge(7, 10, 10);
+        graph.addEdge(7, 11, 10);
+        graph.addEdge(7, 12, 5);
+        TreeMatching treeMatching = new TreeMatching(graph);
+        assertEquals(100, treeMatching.getMaxMatching(0, -1));
+    }
+
+    @Test
+    void testMaxMatchingForTreeWithVariedEdgeWeights() {
+        graph.addEdge(0, 1, 20);
+        graph.addEdge(0, 2, 30);
+        graph.addEdge(0, 3, 40);
+        graph.addEdge(0, 4, 50);
+        graph.addEdge(1, 5, 20);
+        graph.addEdge(2, 6, 20);
+        graph.addEdge(3, 7, 30);
+        graph.addEdge(5, 8, 10);
+        graph.addEdge(5, 9, 20);
+        graph.addEdge(7, 10, 10);
+        graph.addEdge(4, 11, 50);
+        graph.addEdge(4, 12, 20);
+        TreeMatching treeMatching = new TreeMatching(graph);
+        assertEquals(140, treeMatching.getMaxMatching(0, -1));
+    }
+
+    @Test
+    void emptyTree() {
+        TreeMatching treeMatching = new TreeMatching(graph);
+        assertEquals(0, treeMatching.getMaxMatching(0, -1));
+    }
+
+    @Test
+    void testSingleNodeTree() {
+        UndirectedAdjacencyListGraph singleNodeGraph = new UndirectedAdjacencyListGraph();
+        singleNodeGraph.addNode();
+
+        TreeMatching treeMatching = new TreeMatching(singleNodeGraph);
+        assertEquals(0, treeMatching.getMaxMatching(0, -1));
+    }
+
+    @Test
+    void testLinearTree() {
+        graph.addEdge(0, 1, 10);
+        graph.addEdge(1, 2, 20);
+        graph.addEdge(2, 3, 30);
+        graph.addEdge(3, 4, 40);
+
+        TreeMatching treeMatching = new TreeMatching(graph);
+        assertEquals(60, treeMatching.getMaxMatching(0, -1));
+    }
+
+    @Test
+    void testStarShapedTree() {
+        graph.addEdge(0, 1, 15);
+        graph.addEdge(0, 2, 25);
+        graph.addEdge(0, 3, 35);
+        graph.addEdge(0, 4, 45);
+
+        TreeMatching treeMatching = new TreeMatching(graph);
+        assertEquals(45, treeMatching.getMaxMatching(0, -1));
+    }
+
+    @Test
+    void testUnbalancedTree() {
+        graph.addEdge(0, 1, 10);
+        graph.addEdge(0, 2, 20);
+        graph.addEdge(1, 3, 30);
+        graph.addEdge(2, 4, 40);
+        graph.addEdge(4, 5, 50);
+
+        TreeMatching treeMatching = new TreeMatching(graph);
+        assertEquals(100, treeMatching.getMaxMatching(0, -1));
+    }
+}