Enhance docs, add tests in MinHeap (TheAlgorithms#5985)

Author: Hardvan

DIRECTORY.md

@@ -871,6 +871,7 @@
               * [LeftistHeapTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/heaps/LeftistHeapTest.java)
               * [MedianFinderTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/heaps/MedianFinderTest.java)
               * [MergeKSortedArraysTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/heaps/MergeKSortedArraysTest.java)
+              * [MinHeapTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/heaps/MinHeapTest.java)
               * [MinPriorityQueueTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/heaps/MinPriorityQueueTest.java)
             * lists
               * [CircleLinkedListTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/lists/CircleLinkedListTest.java)

src/main/java/com/thealgorithms/datastructures/heaps/Heap.java

@@ -40,5 +40,5 @@ public interface Heap {
      * @param elementIndex int containing the position in the heap of the
      * element to be deleted.
      */
-    void deleteElement(int elementIndex);
+    void deleteElement(int elementIndex) throws EmptyHeapException;
 }

src/main/java/com/thealgorithms/datastructures/heaps/HeapElement.java

@@ -167,4 +167,8 @@ public int hashCode() {
         result += (additionalInfo != null) ? additionalInfo.hashCode() : 0;
         return result;
     }
+
+    public String getValue() {
+        return additionalInfo.toString();
+    }
 }

src/main/java/com/thealgorithms/datastructures/heaps/MinHeap.java

@@ -4,122 +4,268 @@
 import java.util.List;
 
 /**
- * Heap tree where a node's key is higher than or equal to its parent's and
- * lower than or equal to its children's.
+ * A Min Heap implementation where each node's key is lower than or equal to its children's keys.
+ * This data structure provides O(log n) time complexity for insertion and deletion operations,
+ * and O(1) for retrieving the minimum element.
+ *
+ * Properties:
+ * 1. Complete Binary Tree
+ * 2. Parent node's key ≤ Children nodes' keys
+ * 3. Root contains the minimum element
+ *
+ * Example usage:
+ * ```java
+ * List<HeapElement> elements = Arrays.asList(
+ *     new HeapElement(5, "Five"),
+ *     new HeapElement(2, "Two")
+ * );
+ * MinHeap heap = new MinHeap(elements);
+ * heap.insertElement(new HeapElement(1, "One"));
+ * HeapElement min = heap.getElement(); // Returns and removes the minimum element
+ * ```
  *
  * @author Nicolas Renard
  */
 public class MinHeap implements Heap {
 
     private final List<HeapElement> minHeap;
 
+    /**
+     * Constructs a new MinHeap from a list of elements.
+     * Null elements in the input list are ignored with a warning message.
+     *
+     * @param listElements List of HeapElement objects to initialize the heap
+     * @throws IllegalArgumentException if the input list is null
+     */
     public MinHeap(List<HeapElement> listElements) {
+        if (listElements == null) {
+            throw new IllegalArgumentException("Input list cannot be null");
+        }
+
         minHeap = new ArrayList<>();
+
+        // Safe initialization: directly add elements first
         for (HeapElement heapElement : listElements) {
             if (heapElement != null) {
-                insertElement(heapElement);
+                minHeap.add(heapElement);
             } else {
                 System.out.println("Null element. Not added to heap");
             }
         }
+
+        // Heapify the array bottom-up
+        for (int i = minHeap.size() / 2; i >= 0; i--) {
+            heapifyDown(i + 1);
+        }
+
         if (minHeap.isEmpty()) {
             System.out.println("No element has been added, empty heap.");
         }
     }
 
-    // Get the element at a given index. The key for the list is equal to index value - 1
+    /**
+     * Retrieves the element at the specified index without removing it.
+     * Note: The index is 1-based for consistency with heap operations.
+     *
+     * @param elementIndex 1-based index of the element to retrieve
+     * @return HeapElement at the specified index
+     * @throws IndexOutOfBoundsException if the index is invalid
+     */
     public HeapElement getElement(int elementIndex) {
         if ((elementIndex <= 0) || (elementIndex > minHeap.size())) {
-            throw new IndexOutOfBoundsException("Index out of heap range");
+            throw new IndexOutOfBoundsException("Index " + elementIndex + " is out of heap range [1, " + minHeap.size() + "]");
         }
         return minHeap.get(elementIndex - 1);
     }
 
-    // Get the key of the element at a given index
+    /**
+     * Retrieves the key value of an element at the specified index.
+     *
+     * @param elementIndex 1-based index of the element
+     * @return double value representing the key
+     * @throws IndexOutOfBoundsException if the index is invalid
+     */
     private double getElementKey(int elementIndex) {
         if ((elementIndex <= 0) || (elementIndex > minHeap.size())) {
-            throw new IndexOutOfBoundsException("Index out of heap range");
+            throw new IndexOutOfBoundsException("Index " + elementIndex + " is out of heap range [1, " + minHeap.size() + "]");
         }
-
         return minHeap.get(elementIndex - 1).getKey();
     }
 
-    // Swaps two elements in the heap
+    /**
+     * Swaps two elements in the heap.
+     *
+     * @param index1 1-based index of first element
+     * @param index2 1-based index of second element
+     */
     private void swap(int index1, int index2) {
         HeapElement temporaryElement = minHeap.get(index1 - 1);
         minHeap.set(index1 - 1, minHeap.get(index2 - 1));
         minHeap.set(index2 - 1, temporaryElement);
     }
 
-    // Toggle an element up to its right place as long as its key is lower than its parent's
+    /**
+     * Maintains heap properties by moving an element down the heap.
+     * Used specifically during initialization.
+     *
+     * @param elementIndex 1-based index of the element to heapify
+     */
+    private void heapifyDown(int elementIndex) {
+        int smallest = elementIndex - 1; // Convert to 0-based index
+        int leftChild = 2 * elementIndex - 1;
+        int rightChild = 2 * elementIndex;
+
+        // Check if left child is smaller than root
+        if (leftChild < minHeap.size() && minHeap.get(leftChild).getKey() < minHeap.get(smallest).getKey()) {
+            smallest = leftChild;
+        }
+
+        // Check if right child is smaller than smallest so far
+        if (rightChild < minHeap.size() && minHeap.get(rightChild).getKey() < minHeap.get(smallest).getKey()) {
+            smallest = rightChild;
+        }
+
+        // If smallest is not root
+        if (smallest != elementIndex - 1) {
+            HeapElement swap = minHeap.get(elementIndex - 1);
+            minHeap.set(elementIndex - 1, minHeap.get(smallest));
+            minHeap.set(smallest, swap);
+
+            // Recursively heapify the affected sub-tree
+            heapifyDown(smallest + 1); // Convert back to 1-based index
+        }
+    }
+
+    /**
+     * Moves an element up the heap until heap properties are satisfied.
+     * This operation is called after insertion to maintain heap properties.
+     *
+     * @param elementIndex 1-based index of the element to move up
+     */
     private void toggleUp(int elementIndex) {
+        if (elementIndex <= 1) {
+            return;
+        }
+
         double key = minHeap.get(elementIndex - 1).getKey();
-        while (getElementKey((int) Math.floor(elementIndex / 2.0) + 1) > key) {
-            swap(elementIndex, (int) Math.floor(elementIndex / 2.0));
-            elementIndex = (int) Math.floor(elementIndex / 2.0);
+        int parentIndex = (int) Math.floor(elementIndex / 2.0);
+
+        while (elementIndex > 1 && getElementKey(parentIndex) > key) {
+            swap(elementIndex, parentIndex);
+            elementIndex = parentIndex;
+            parentIndex = (int) Math.floor(elementIndex / 2.0);
         }
     }
 
-    // Toggle an element down to its right place as long as its key is higher
-    // than any of its children's
+    /**
+     * Moves an element down the heap until heap properties are satisfied.
+     * This operation is called after deletion to maintain heap properties.
+     *
+     * @param elementIndex 1-based index of the element to move down
+     */
     private void toggleDown(int elementIndex) {
         double key = minHeap.get(elementIndex - 1).getKey();
-        boolean wrongOrder = (key > getElementKey(elementIndex * 2)) || (key > getElementKey(Math.min(elementIndex * 2, minHeap.size())));
-        while ((2 * elementIndex <= minHeap.size()) && wrongOrder) {
-            // Check whether it shall swap the element with its left child or its right one if any.
-            if ((2 * elementIndex < minHeap.size()) && (getElementKey(elementIndex * 2 + 1) < getElementKey(elementIndex * 2))) {
-                swap(elementIndex, 2 * elementIndex + 1);
-                elementIndex = 2 * elementIndex + 1;
-            } else {
-                swap(elementIndex, 2 * elementIndex);
-                elementIndex = 2 * elementIndex;
+        int size = minHeap.size();
+
+        while (true) {
+            int smallest = elementIndex;
+            int leftChild = 2 * elementIndex;
+            int rightChild = 2 * elementIndex + 1;
+
+            if (leftChild <= size && getElementKey(leftChild) < key) {
+                smallest = leftChild;
+            }
+
+            if (rightChild <= size && getElementKey(rightChild) < getElementKey(smallest)) {
+                smallest = rightChild;
+            }
+
+            if (smallest == elementIndex) {
+                break;
             }
-            wrongOrder = (key > getElementKey(elementIndex * 2)) || (key > getElementKey(Math.min(elementIndex * 2, minHeap.size())));
+
+            swap(elementIndex, smallest);
+            elementIndex = smallest;
         }
     }
 
-    private HeapElement extractMin() {
-        HeapElement result = minHeap.get(0);
-        deleteElement(0);
+    /**
+     * Extracts and returns the minimum element from the heap.
+     *
+     * @return HeapElement with the lowest key
+     * @throws EmptyHeapException if the heap is empty
+     */
+    private HeapElement extractMin() throws EmptyHeapException {
+        if (minHeap.isEmpty()) {
+            throw new EmptyHeapException("Cannot extract from empty heap");
+        }
+        HeapElement result = minHeap.getFirst();
+        deleteElement(1);
         return result;
     }
 
+    /**
+     * {@inheritDoc}
+     */
     @Override
-    public final void insertElement(HeapElement element) {
+    public void insertElement(HeapElement element) {
+        if (element == null) {
+            throw new IllegalArgumentException("Cannot insert null element");
+        }
         minHeap.add(element);
         toggleUp(minHeap.size());
     }
 
+    /**
+     * {@inheritDoc}
+     */
     @Override
-    public void deleteElement(int elementIndex) {
+    public void deleteElement(int elementIndex) throws EmptyHeapException {
         if (minHeap.isEmpty()) {
-            try {
-                throw new EmptyHeapException("Attempt to delete an element from an empty heap");
-            } catch (EmptyHeapException e) {
-                e.printStackTrace();
-            }
+            throw new EmptyHeapException("Cannot delete from empty heap");
         }
         if ((elementIndex > minHeap.size()) || (elementIndex <= 0)) {
-            throw new IndexOutOfBoundsException("Index out of heap range");
-        }
-        // The last element in heap replaces the one to be deleted
-        minHeap.set(elementIndex - 1, getElement(minHeap.size()));
-        minHeap.remove(minHeap.size());
-        // Shall the new element be moved up...
-        if (getElementKey(elementIndex) < getElementKey((int) Math.floor(elementIndex / 2.0))) {
-            toggleUp(elementIndex);
-        } // ... or down ?
-        else if (((2 * elementIndex <= minHeap.size()) && (getElementKey(elementIndex) > getElementKey(elementIndex * 2))) || ((2 * elementIndex < minHeap.size()) && (getElementKey(elementIndex) > getElementKey(elementIndex * 2)))) {
-            toggleDown(elementIndex);
+            throw new IndexOutOfBoundsException("Index " + elementIndex + " is out of heap range [1, " + minHeap.size() + "]");
+        }
+
+        // Replace with last element and remove last position
+        minHeap.set(elementIndex - 1, minHeap.getLast());
+        minHeap.removeLast();
+
+        // No need to toggle if we just removed the last element
+        if (!minHeap.isEmpty() && elementIndex <= minHeap.size()) {
+            // Determine whether to toggle up or down
+            if (elementIndex > 1 && getElementKey(elementIndex) < getElementKey((int) Math.floor(elementIndex / 2.0))) {
+                toggleUp(elementIndex);
+            } else {
+                toggleDown(elementIndex);
+            }
         }
     }
 
+    /**
+     * {@inheritDoc}
+     */
     @Override
     public HeapElement getElement() throws EmptyHeapException {
-        try {
-            return extractMin();
-        } catch (Exception e) {
-            throw new EmptyHeapException("Heap is empty. Error retrieving element", e);
-        }
+        return extractMin();
+    }
+
+    /**
+     * Returns the current size of the heap.
+     *
+     * @return number of elements in the heap
+     */
+    public int size() {
+        return minHeap.size();
+    }
+
+    /**
+     * Checks if the heap is empty.
+     *
+     * @return true if the heap contains no elements
+     */
+    public boolean isEmpty() {
+        return minHeap.isEmpty();
     }
 }