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Feature/centroid decomposition (#7086)
* feat: Add Centroid Decomposition for trees (#7054) - Implement CentroidDecomposition with O(N log N) construction - Add CentroidTree class with parent tracking and query methods - Include buildFromEdges helper for easy tree construction - Add comprehensive test suite with 20+ test cases - Cover edge cases, validation, and various tree structures Closes #7054 * feat: Add Centroid Decomposition for trees (#7054) - Implement CentroidDecomposition with O(N log N) construction - Add CentroidTree class with parent tracking and query methods - Include buildFromEdges helper for easy tree construction - Add comprehensive test suite with 20+ test cases - Cover edge cases, validation, and various tree structures Closes #7054 * fix: Remove trailing whitespace from CentroidDecompositionTest * fix: Remove trailing whitespace and add newlines at end of files * fix: Format code to comply with clang-format and checkstyle requirements * style: Fix lambda formatting in test assertions - Change single-line lambdas to multi-line format - Align with repository code style guidelines * style: Apply clang-format to match repository style guide - Format code according to .clang-format configuration - Use single-line lambdas as allowed by AllowShortLambdasOnASingleLine: All - Apply 4-space indentation - Ensure proper line endings
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‎src/main/java/com/thealgorithms/datastructures/trees/CentroidDecomposition.java‎

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package com.thealgorithms.datastructures.trees;
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import java.util.ArrayList;
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import java.util.Arrays;
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import java.util.List;
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/**
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* Centroid Decomposition is a divide-and-conquer technique for trees.
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* It recursively partitions a tree by finding centroids - nodes whose removal
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* creates balanced subtrees (each with at most N/2 nodes).
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*
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* <p>
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* Time Complexity: O(N log N) for construction
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* Space Complexity: O(N)
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*
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* <p>
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* Applications:
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* - Distance queries on trees
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* - Path counting problems
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* - Nearest neighbor searches
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*
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* @see <a href="https://en.wikipedia.org/wiki/Centroid_decomposition">Centroid Decomposition</a>
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* @see <a href="https://codeforces.com/blog/entry/81661">Centroid Decomposition Tutorial</a>
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* @author lens161
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*/
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public final class CentroidDecomposition {
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private CentroidDecomposition() {
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}
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/**
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* Represents the centroid tree structure.
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*/
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public static final class CentroidTree {
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private final int n;
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private final List<List<Integer>> adj;
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private final int[] parent;
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private final int[] subtreeSize;
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private final boolean[] removed;
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private int root;
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/**
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* Constructs a centroid tree from an adjacency list.
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*
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* @param adj adjacency list representation of the tree (0-indexed)
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* @throws IllegalArgumentException if tree is empty or null
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*/
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public CentroidTree(List<List<Integer>> adj) {
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if (adj == null || adj.isEmpty()) {
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throw new IllegalArgumentException("Tree cannot be empty or null");
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}
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this.n = adj.size();
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this.adj = adj;
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this.parent = new int[n];
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this.subtreeSize = new int[n];
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this.removed = new boolean[n];
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Arrays.fill(parent, -1);
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// Build centroid tree starting from node 0
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this.root = decompose(0, -1);
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}
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/**
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* Recursively builds the centroid tree.
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*
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* @param u current node
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* @param p parent in centroid tree
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* @return centroid of current component
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*/
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private int decompose(int u, int p) {
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int size = getSubtreeSize(u, -1);
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int centroid = findCentroid(u, -1, size);
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removed[centroid] = true;
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parent[centroid] = p;
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// Recursively decompose each subtree
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for (int v : adj.get(centroid)) {
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if (!removed[v]) {
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decompose(v, centroid);
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}
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}
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return centroid;
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}
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/**
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* Calculates subtree size from node u.
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*
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* @param u current node
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* @param p parent node (-1 for root)
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* @return size of subtree rooted at u
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*/
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private int getSubtreeSize(int u, int p) {
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subtreeSize[u] = 1;
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for (int v : adj.get(u)) {
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if (v != p && !removed[v]) {
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subtreeSize[u] += getSubtreeSize(v, u);
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}
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}
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return subtreeSize[u];
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}
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/**
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* Finds the centroid of a subtree.
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* A centroid is a node whose removal creates components with size &lt;= totalSize/2.
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*
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* @param u current node
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* @param p parent node
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* @param totalSize total size of current component
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* @return centroid node
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*/
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private int findCentroid(int u, int p, int totalSize) {
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for (int v : adj.get(u)) {
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if (v != p && !removed[v] && subtreeSize[v] > totalSize / 2) {
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return findCentroid(v, u, totalSize);
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}
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}
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return u;
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}
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/**
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* Gets the parent of a node in the centroid tree.
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*
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* @param node the node
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* @return parent node in centroid tree, or -1 if root
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*/
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public int getParent(int node) {
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if (node < 0 || node >= n) {
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throw new IllegalArgumentException("Invalid node: " + node);
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}
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return parent[node];
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}
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/**
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* Gets the root of the centroid tree.
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*
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* @return root node
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*/
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public int getRoot() {
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return root;
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}
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/**
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* Gets the number of nodes in the tree.
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*
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* @return number of nodes
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*/
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public int size() {
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return n;
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}
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/**
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* Returns the centroid tree structure as a string.
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* Format: node -&gt; parent (or ROOT for root node)
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*
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* @return string representation
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*/
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@Override
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public String toString() {
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StringBuilder sb = new StringBuilder("Centroid Tree:\n");
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for (int i = 0; i < n; i++) {
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sb.append("Node ").append(i).append(" -> ");
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if (parent[i] == -1) {
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sb.append("ROOT");
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} else {
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sb.append("Parent ").append(parent[i]);
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}
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sb.append("\n");
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}
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return sb.toString();
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}
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}
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/**
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* Creates a centroid tree from an edge list.
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*
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* @param n number of nodes (0-indexed: 0 to n-1)
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* @param edges list of edges where each edge is [u, v]
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* @return CentroidTree object
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* @throws IllegalArgumentException if n &lt;= 0 or edges is invalid
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*/
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public static CentroidTree buildFromEdges(int n, int[][] edges) {
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if (n <= 0) {
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throw new IllegalArgumentException("Number of nodes must be positive");
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}
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if (edges == null) {
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throw new IllegalArgumentException("Edges cannot be null");
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}
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if (edges.length != n - 1) {
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throw new IllegalArgumentException("Tree must have exactly n-1 edges");
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}
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List<List<Integer>> adj = new ArrayList<>();
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for (int i = 0; i < n; i++) {
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adj.add(new ArrayList<>());
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}
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for (int[] edge : edges) {
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if (edge.length != 2) {
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throw new IllegalArgumentException("Each edge must have exactly 2 nodes");
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}
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int u = edge[0];
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int v = edge[1];
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if (u < 0 || u >= n || v < 0 || v >= n) {
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throw new IllegalArgumentException("Invalid node in edge: [" + u + ", " + v + "]");
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}
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adj.get(u).add(v);
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adj.get(v).add(u);
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}
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return new CentroidTree(adj);
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}
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}

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