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58 lines
1.6 KiB
Java
58 lines
1.6 KiB
Java
import java.util.*;
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import java.util.stream.Stream;
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// https://sai16vicky.wordpress.com/2014/11/01/divide-and-conquer-on-trees-centroid-decomposition/
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public class CentroidDecomposition {
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static void calcSizes(List<Integer>[] tree, int[] size, boolean[] deleted, int u, int p) {
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size[u] = 1;
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for (int v : tree[u]) {
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if (v == p || deleted[v]) continue;
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calcSizes(tree, size, deleted, v, u);
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size[u] += size[v];
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}
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}
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static int findTreeCentroid(List<Integer>[] tree, int[] size, boolean[] deleted, int u, int p, int vertices) {
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for (int v : tree[u]) {
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if (v == p || deleted[v]) continue;
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if (size[v] > vertices / 2) {
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return findTreeCentroid(tree, size, deleted, v, u, vertices);
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}
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}
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return u;
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}
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static void decompose(List<Integer>[] tree, int[] size, boolean[] deleted, int u, int total) {
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calcSizes(tree, size, deleted, u, -1);
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int centroid = findTreeCentroid(tree, size, deleted, u, -1, total);
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calcSizes(tree, size, deleted, centroid, -1);
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deleted[centroid] = true;
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System.out.println(centroid);
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for (int v : tree[centroid]) {
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if (deleted[v]) continue;
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decompose(tree, size, deleted, v, size[v]);
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}
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}
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public static void centroidDecomposition(List<Integer>[] tree) {
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int n = tree.length;
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decompose(tree, new int[n], new boolean[n], 0, n);
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}
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// Usage example
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public static void main(String[] args) {
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int n = 4;
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List<Integer>[] tree = Stream.generate(ArrayList::new).limit(n).toArray(List[]::new);
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tree[3].add(0);
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tree[0].add(3);
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tree[3].add(1);
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tree[1].add(3);
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tree[3].add(2);
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tree[2].add(3);
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centroidDecomposition(tree);
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}
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}
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