style: enable EqualsHashCode in checkstyle (#5157)

checkstyle.xml

@@ -161,7 +161,7 @@
     <!-- Checks for common coding problems               -->
     <!-- See https://checkstyle.org/checks/coding/index.html -->
     <module name="EmptyStatement"/>
-    <!-- TODO <module name="EqualsHashCode"/> -->
+    <module name="EqualsHashCode"/>
     <!-- TODO <module name="HiddenField"/> -->
     <module name="IllegalInstantiation"/>
     <!-- TODO <module name="InnerAssignment"/> -->

src/main/java/com/thealgorithms/datastructures/trees/KDTree.java

@@ -81,6 +81,11 @@ public class KDTree {
             return false;
         }
 
+        @Override
+        public int hashCode() {
+            return Arrays.hashCode(coordinates);
+        }
+
         @Override
         public String toString() {
             return Arrays.toString(coordinates);