throw exception for invalid startNode, add tests, clean up

Author: lennart

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

@@ -12,31 +12,28 @@ public class CentroidDecomposition {
     private int startingNode;
     private int N;
 
+    @SuppressWarnings("unchecked")
     public CentroidDecomposition(int n, int startingNode){
         tree = new ArrayList[n];
         centroidTree = new ArrayList[n];
         N = n;
-        for (int i = 0; i < centroidTree.length; i++) centroidTree[i] = new ArrayList<>();
-        for (int i = 0; i < tree.length; i++) tree[i] = new ArrayList<>();
         centroidMarked = new boolean[n];
         centroidParent = new int[n];
-        startingNode = (int)(Math.random() * n+1);
+        if (startingNode < 0 || startingNode > n-1){
+            throw new IllegalArgumentException("Starting node must be in range 0.." + (n - 1) + " but got " + startingNode);
+        }
         for(int i = 0; i<n; i++){
             tree[i] = new ArrayList<>();
+            centroidTree[i] = new ArrayList<>();
         }
     }
+
     public CentroidDecomposition(int n){
-        tree = new ArrayList[n];
-        centroidTree = new ArrayList[n];
-        centroidMarked = new boolean[n];
-        centroidParent = new int[n];
-        startingNode = (this.startingNode == -1) ? (int)(Math.random() * n+1) : this.startingNode;
-        N = n;
-        for (int i = 0; i < centroidTree.length; i++) centroidTree[i] = new ArrayList<>();
-        for (int i = 0; i < tree.length; i++) tree[i] = new ArrayList<>();
-        for(int i = 0; i<n; i++){
-            tree[i] = new ArrayList<>();
-        }
+        this(n, (int)(Math.random() * n));
+    }
+
+    public int getStartingNode(){
+        return startingNode;
     }
 
     public List<Integer>[] getCentroidTree(){
@@ -54,7 +51,6 @@ private void addEdgeCTree(int u, int v){
         centroidParent[v] = u;
     }
 
-
     public void findSubtreeSizes(int src, boolean[] visited, int[] subtreeSizes){
         // dfs traversal to find size of subtree rooted at src
         visited[src] = true;
@@ -87,14 +83,12 @@ public void findCentroid(int src, int previousCentroid){
 
         if (centroidMarked[src]){
             if (src != startingNode && src != previousCentroid) addEdgeCTree(previousCentroid, src); 
-            // centroidTree[previousCentroid].add(src);
             for (int node : tree[src]){
                 if (!centroidMarked[node])
                     findCentroid(node, src);                
             }
         }
         else{
-            // pick one of the children of the root for next check
             int nextLargestSubtree = tree[src].getFirst();
             for (int node : tree[src]){
                 if (subtreeSizes[node] >= subtreeSizes[nextLargestSubtree]) 
@@ -105,7 +99,7 @@ public void findCentroid(int src, int previousCentroid){
     }
 
     public static void main(String[] args) {
-        CentroidDecomposition cd = new CentroidDecomposition(16, -1);
+        CentroidDecomposition cd = new CentroidDecomposition(16);
         cd.addEdge(0, 1);
         cd.addEdge(0, 2);
         cd.addEdge(0, 3);
@@ -126,7 +120,7 @@ public static void main(String[] args) {
         int[] subtreeSizes = new int[16];
 
         // int start = cd.startingNode;
-        int src = (int) ((Math.random() * (15)) + 0);
+        int src = (int)(Math.random() * 15);
         System.out.println("src= " + src);
 
         cd.findCentroid(src, src);

src/test/java/com/thealgorithms/tree/CentroidDecompositionTest.java

@@ -1,12 +1,9 @@
 package com.thealgorithms.tree;
 
-import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 import static org.junit.jupiter.api.Assertions.assertEquals;
-import static org.junit.jupiter.api.Assertions.assertTrue;
-import static org.mockito.Answers.values;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import java.util.ArrayList;
-import java.util.Arrays;
 import java.util.List;
 
 import org.junit.jupiter.api.BeforeEach;
@@ -37,7 +34,8 @@ void setUp(){
         cd.addEdge(14, 15);
 
         /*
-         *          0
+         * Initial Tree:
+         *           0
                 /   |    \
                1    2     3
               / \  / \     \
@@ -52,7 +50,7 @@ void setUp(){
                            15
 
 
-
+         * centroid Tree:
                     0
              /      |       \
             1      11        8
@@ -74,12 +72,18 @@ void testFindSubtreeSizes(){
         assertEquals(subtreeSizes[0], 12);
     }
 
+    @Test
+    void IllegalArgumentThrows(){
+        assertThrows(IllegalArgumentException.class, () -> {
+            new CentroidDecomposition(10, 99);
+        });
+    }
+
     @RepeatedTest(100)
     void testBuildCentroidTree(){
-        int src = (int) ((Math.random() * (15)) + 0);
-        cd.findCentroid(src, src);
+        int start = cd.getStartingNode();
+        System.out.println(start);
         List<Integer>[] centroidTree = cd.getCentroidTree();
-        
         List<Integer> correct = new ArrayList<Integer>();
         correct.add(0);
         correct.add(3);