From d14a5d1eed14ca9aee01e2f775c7a3128ef20659 Mon Sep 17 00:00:00 2001
From: Artem Boiarshinov <54187376+Boiarshinov@users.noreply.github.com>
Date: Wed, 22 Jun 2022 16:56:35 +0300
Subject: [PATCH] Fix SkipList remove operation (#3160)

---
 .../datastructures/lists/SkipList.java         |  4 +++-
 .../datastructures/lists/SkipListTest.java     | 18 ++++++++++++++++--
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/src/main/java/com/thealgorithms/datastructures/lists/SkipList.java b/src/main/java/com/thealgorithms/datastructures/lists/SkipList.java
index 5cd14f9dd..259406e22 100644
--- a/src/main/java/com/thealgorithms/datastructures/lists/SkipList.java
+++ b/src/main/java/com/thealgorithms/datastructures/lists/SkipList.java
@@ -117,7 +117,9 @@ public class SkipList<E extends Comparable<E>> {
         }
         for (int i = 0; i <= layer; i++) {
             current.previous(i).setNext(i, current.next(i));
-            current.next(i).setPrevious(i, current.previous(i));
+            if (current.next(i) != null) {
+                current.next(i).setPrevious(i, current.previous(i));
+            }
         }
         size--;
     }
diff --git a/src/test/java/com/thealgorithms/datastructures/lists/SkipListTest.java b/src/test/java/com/thealgorithms/datastructures/lists/SkipListTest.java
index b26de0c55..1bbe11527 100644
--- a/src/test/java/com/thealgorithms/datastructures/lists/SkipListTest.java
+++ b/src/test/java/com/thealgorithms/datastructures/lists/SkipListTest.java
@@ -42,12 +42,26 @@ class SkipListTest {
     }
 
     @Test
-    void remove() {
+    void removeFromHead() {
         SkipList<String> skipList = createSkipList();
+        String mostLeftElement = skipList.get(0);
         int initialSize = skipList.size();
         print(skipList);
 
-        skipList.remove("a");
+        skipList.remove(mostLeftElement);
+
+        print(skipList);
+        assertEquals(initialSize - 1, skipList.size());
+    }
+
+    @Test
+    void removeFromTail() {
+        SkipList<String> skipList = createSkipList();
+        String mostRightValue = skipList.get(skipList.size() - 1);
+        int initialSize = skipList.size();
+        print(skipList);
+
+        skipList.remove(mostRightValue);
 
         print(skipList);
         assertEquals(initialSize - 1, skipList.size());