[BAEL-7557] SkipList-implementation-in-Java (#16253)

algorithms-modules/algorithms-miscellaneous-8/src/main/java/com/baeldung/algorithms/skiplist/Node.java

@@ -0,0 +1,11 @@
+package com.baeldung.algorithms.skiplist;
+
+class Node {
+    int value;
+    Node[] forward; // array to hold references to different levels
+
+    public Node(int value, int level) {
+        this.value = value;
+        this.forward = new Node[level + 1]; // level + 1 because level is 0-based
+    }
+}

algorithms-modules/algorithms-miscellaneous-8/src/main/java/com/baeldung/algorithms/skiplist/SkipList.java

@@ -0,0 +1,92 @@
+package com.baeldung.algorithms.skiplist;
+
+
+import java.util.Random;
+
+public class SkipList {
+    private Node head;
+    private int maxLevel;
+    private int level;
+    private Random random;
+
+    public SkipList() {
+        maxLevel = 16; // maximum number of levels
+        level = 0; // current level of SkipList
+        head = new Node(Integer.MIN_VALUE, maxLevel);
+        random = new Random();
+    }
+
+    public void insert(int value) {
+        Node[] update = new Node[maxLevel + 1];
+        Node current = this.head;
+
+        for (int i = level; i >= 0; i--) {
+            while (current.forward[i] != null && current.forward[i].value < value) {
+                current = current.forward[i];
+            }
+            update[i] = current;
+        }
+
+        current = current.forward[0];
+
+        if (current == null || current.value != value) {
+            int lvl = randomLevel();
+
+            if (lvl > level) {
+                for (int i = level + 1; i <= lvl; i++) {
+                    update[i] = head;
+                }
+                level = lvl;
+            }
+
+            Node newNode = new Node(value, lvl);
+            for (int i = 0; i <= lvl; i++) {
+                newNode.forward[i] = update[i].forward[i];
+                update[i].forward[i] = newNode;
+            }
+        }
+    }
+
+    public boolean search(int value) {
+        Node current = this.head;
+        for (int i = level; i >= 0; i--) {
+            while (current.forward[i] != null && current.forward[i].value < value) {
+                current = current.forward[i];
+            }
+        }
+        current = current.forward[0];
+        return current != null && current.value == value;
+    }
+
+    public void delete(int value) {
+        Node[] update = new Node[maxLevel + 1];
+        Node current = this.head;
+
+        for (int i = level; i >= 0; i--) {
+            while (current.forward[i] != null && current.forward[i].value < value) {
+                current = current.forward[i];
+            }
+            update[i] = current;
+        }
+        current = current.forward[0];
+
+        if (current != null && current.value == value) {
+            for (int i = 0; i <= level; i++) {
+                if (update[i].forward[i] != current) break;
+                update[i].forward[i] = current.forward[i];
+            }
+
+            while (level > 0 && head.forward[level] == null) {
+                level--;
+            }
+        }
+    }
+
+    private int randomLevel() {
+        int lvl = 0;
+        while (lvl < maxLevel && random.nextDouble() < 0.5) {
+            lvl++;
+        }
+        return lvl;
+    }
+}

algorithms-modules/algorithms-miscellaneous-8/src/test/java/com/baeldung/algorithms/skiplist/SkipListUnitTest.java

@@ -0,0 +1,34 @@
+package com.baeldung.algorithms.skiplist;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class SkipListUnitTest {
+
+    @Test
+    public void givenSkipList_WhenInsert_ThenSearchFound() {
+        SkipList skipList = new SkipList();
+
+        skipList.insert(3);
+        assertTrue(skipList.search(3), "Should find 3");
+
+        assertFalse(skipList.search(99), "Should not find 99");
+    }
+
+    @Test
+    public void givenSkipList_WhenDeleteElement_ThenRemoveFromList() {
+        SkipList skipList = new SkipList();
+
+        skipList.insert(3);
+        skipList.insert(7);
+
+        skipList.delete(3);
+        assertFalse(skipList.search(3), "3 should have been deleted");
+
+        skipList.delete(99);
+        assertTrue(skipList.search(7), "7 should still exist");
+    }
+
+}