Add Heavy-Light Decomposition (HLD) (#6169)

src/main/java/com/thealgorithms/tree/HeavyLightDecomposition.java

@@ -0,0 +1,157 @@
+package com.thealgorithms.tree;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Heavy-Light Decomposition (HLD) implementation in Java.
+ * HLD is used to efficiently handle path queries on trees, such as maximum,
+ * sum, or updates. It decomposes the tree into heavy and light chains,
+ * enabling queries in O(log N) time.
+ * Wikipedia Reference: https://en.wikipedia.org/wiki/Heavy-light_decomposition
+ * Author: Nithin U.
+ * Github: https://github.com/NithinU2802
+ */
+
+public class HeavyLightDecomposition {
+    private List<List<Integer>> tree;
+    private int[] parent;
+    private int[] depth;
+    private int[] subtreeSize;
+    private int[] chainHead;
+    private int[] position;
+    private int[] nodeValue;
+    private int[] segmentTree;
+    private int positionIndex;
+
+    public HeavyLightDecomposition(int n) {
+        tree = new ArrayList<>();
+        for (int i = 0; i <= n; i++) {
+            tree.add(new ArrayList<>());
+        }
+        parent = new int[n + 1];
+        depth = new int[n + 1];
+        subtreeSize = new int[n + 1];
+        chainHead = new int[n + 1];
+        position = new int[n + 1];
+        nodeValue = new int[n + 1];
+        segmentTree = new int[4 * (n + 1)];
+        for (int i = 0; i <= n; i++) {
+            chainHead[i] = -1;
+        }
+        positionIndex = 0;
+    }
+
+    public int getPosition(int index) {
+        return position[index];
+    }
+
+    public int getPositionIndex() {
+        return positionIndex;
+    }
+
+    public void addEdge(int u, int v) {
+        tree.get(u).add(v);
+        tree.get(v).add(u);
+    }
+
+    private void dfsSize(int node, int parentNode) {
+        parent[node] = parentNode;
+        subtreeSize[node] = 1;
+        for (int child : tree.get(node)) {
+            if (child != parentNode) {
+                depth[child] = depth[node] + 1;
+                dfsSize(child, node);
+                subtreeSize[node] += subtreeSize[child];
+            }
+        }
+    }
+
+    private void decompose(int node, int head) {
+        chainHead[node] = head;
+        position[node] = positionIndex++;
+        int heavyChild = -1;
+        int maxSubtreeSize = -1;
+        for (int child : tree.get(node)) {
+            if (child != parent[node] && subtreeSize[child] > maxSubtreeSize) {
+                heavyChild = child;
+                maxSubtreeSize = subtreeSize[child];
+            }
+        }
+        if (heavyChild != -1) {
+            decompose(heavyChild, head);
+        }
+        for (int child : tree.get(node)) {
+            if (child != parent[node] && child != heavyChild) {
+                decompose(child, child);
+            }
+        }
+    }
+
+    private void buildSegmentTree(int node, int start, int end) {
+        if (start == end) {
+            segmentTree[node] = nodeValue[start];
+            return;
+        }
+        int mid = (start + end) / 2;
+        buildSegmentTree(2 * node, start, mid);
+        buildSegmentTree(2 * node + 1, mid + 1, end);
+        segmentTree[node] = Math.max(segmentTree[2 * node], segmentTree[2 * node + 1]);
+    }
+
+    public void updateSegmentTree(int node, int start, int end, int index, int value) {
+        if (start == end) {
+            segmentTree[node] = value;
+            return;
+        }
+        int mid = (start + end) / 2;
+        if (index <= mid) {
+            updateSegmentTree(2 * node, start, mid, index, value);
+        } else {
+            updateSegmentTree(2 * node + 1, mid + 1, end, index, value);
+        }
+        segmentTree[node] = Math.max(segmentTree[2 * node], segmentTree[2 * node + 1]);
+    }
+
+    public int querySegmentTree(int node, int start, int end, int left, int right) {
+        if (left > end || right < start) {
+            return Integer.MIN_VALUE;
+        }
+        if (left <= start && end <= right) {
+            return segmentTree[node];
+        }
+        int mid = (start + end) / 2;
+        int leftQuery = querySegmentTree(2 * node, start, mid, left, right);
+        int rightQuery = querySegmentTree(2 * node + 1, mid + 1, end, left, right);
+        return Math.max(leftQuery, rightQuery);
+    }
+
+    public int queryMaxInPath(int u, int v) {
+        int result = Integer.MIN_VALUE;
+        while (chainHead[u] != chainHead[v]) {
+            if (depth[chainHead[u]] < depth[chainHead[v]]) {
+                int temp = u;
+                u = v;
+                v = temp;
+            }
+            result = Math.max(result, querySegmentTree(1, 0, positionIndex - 1, position[chainHead[u]], position[u]));
+            u = parent[chainHead[u]];
+        }
+        if (depth[u] > depth[v]) {
+            int temp = u;
+            u = v;
+            v = temp;
+        }
+        result = Math.max(result, querySegmentTree(1, 0, positionIndex - 1, position[u], position[v]));
+        return result;
+    }
+
+    public void initialize(int root, int[] values) {
+        dfsSize(root, -1);
+        decompose(root, root);
+        for (int i = 0; i < values.length; i++) {
+            nodeValue[position[i]] = values[i];
+        }
+        buildSegmentTree(1, 0, positionIndex - 1);
+    }
+}