Add Leftist Heap (#3789)

Co-authored-by: Adrian Paras <aparas@terpmail.umd.edu>

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

@@ -0,0 +1,118 @@
+package com.thealgorithms.datastructures.heaps;
+
+import java.util.ArrayList;
+
+/* 
+ * This is a leftist heap that follows the same operations as a
+ * binary min heap, but may be unbalanced at times and follows a
+ * leftist property, in which the left side is more heavy on the
+ * right based on the null-path length (npl) values.
+ * 
+ * Source: https://iq.opengenus.org/leftist-heap/
+ * 
+ */
+
+public class LeftistHeap {
+	private class Node {
+		private int element, npl;
+		private Node left, right;
+
+		// Node constructor setting the data element and left/right pointers to null
+		private Node(int element) {
+			this.element = element;
+			left = right = null;
+			npl = 0;
+		}
+	}
+
+	private Node root;
+
+	// Constructor
+	public LeftistHeap() {
+		root = null;
+	}
+
+	// Checks if heap is empty
+	public boolean isEmpty() {
+		return root == null;
+	}
+
+	// Resets structure to initial state
+	public void clear() {
+		// We will put head is null
+		root = null;
+	}
+
+	// Merge function that merges the contents of another leftist heap with the
+	// current one
+	public void merge(LeftistHeap h1) {
+		// If the present function is rhs then we ignore the merge
+		root = merge(root, h1.root);
+		h1.root = null;
+	}
+
+	// Function merge with two Nodes a and b
+	public Node merge(Node a, Node b) {
+		if (a == null)
+			return b;
+
+		if (b == null)
+			return a;
+
+		// Violates leftist property, so must do a swap
+		if (a.element > b.element) {
+			Node temp = a;
+			a = b;
+			b = temp;
+		}
+
+		// Now we call the function merge to merge a and b
+		a.right = merge(a.right, b);
+
+		// Violates leftist property so must swap here
+		if (a.left == null) {
+			a.left = a.right;
+			a.right = null;
+		} else {
+			if (a.left.npl < a.right.npl) {
+				Node temp = a.left;
+				a.left = a.right;
+				a.right = temp;
+			}
+			a.npl = a.right.npl + 1;
+		}
+		return a;
+	}
+
+	// Function insert. Uses the merge function to add the data
+	public void insert(int a) {
+		root = merge(new Node(a), root);
+	}
+
+	// Returns and removes the minimum element in the heap
+	public int extract_min() {
+		// If is empty return -1
+		if (isEmpty())
+			return -1;
+
+		int min = root.element;
+		root = merge(root.left, root.right);
+		return min;
+	}
+
+	// Function returning a list of an in order traversal of the data structure
+	public ArrayList<Integer> in_order() {
+		ArrayList<Integer> lst = new ArrayList<>();
+		in_order_aux(root, lst);
+		return new ArrayList<>(lst);
+	}
+
+	// Auxiliary function for in_order
+	private void in_order_aux(Node n, ArrayList<Integer> lst) {
+		if (n == null)
+			return;
+		in_order_aux(n.left, lst);
+		lst.add(n.element);
+		in_order_aux(n.right, lst);
+	}
+}

src/test/java/com/thealgorithms/datastructures/heaps/LeftistHeapTest.java

@@ -0,0 +1,28 @@
+package com.thealgorithms.datastructures.heaps;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class LeftistHeapTest {
+	
+	@Test
+	void testLeftistHeap() {
+		LeftistHeap heap = new LeftistHeap();
+		Assertions.assertTrue(heap.isEmpty());
+		heap.insert(6);
+		Assertions.assertTrue(!heap.isEmpty());
+		heap.insert(2);
+		heap.insert(3);
+		heap.insert(1);
+		heap.in_order();
+		Assertions.assertTrue(heap.in_order().toString().equals("[6, 2, 3, 1]"));
+		Assertions.assertTrue(heap.extract_min() == 1);
+		Assertions.assertTrue(heap.in_order().toString().equals("[6, 2, 3]"));
+		heap.insert(8);
+		heap.insert(12);
+		heap.insert(4);
+		Assertions.assertTrue(heap.in_order().toString().equals("[8, 3, 12, 2, 6, 4]"));
+		heap.clear();
+		Assertions.assertTrue(heap.isEmpty());
+	}
+}