Fix SkipList remove operation (TheAlgorithms#3160)

Boiarshinov · web-flow · commit d14a5d1eed14 · 2022-06-22T21:56:35.000+08:00
diff --git 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 void remove(E 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
@@ -42,12 +42,26 @@ void contains() {
     }
 
     @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());

Original file line number	Diff line number	Diff line change
`@@ -117,7 +117,9 @@ public void remove(E e) {`
`117`	`117`	`}`
`118`	`118`	`for (int i = 0; i <= layer; i++) {`
`119`	`119`	`current.previous(i).setNext(i, current.next(i));`
`120`		`- current.next(i).setPrevious(i, current.previous(i));`
	`120`	`+ if (current.next(i) != null) {`
	`121`	`+ current.next(i).setPrevious(i, current.previous(i));`
	`122`	`+ }`
`121`	`123`	`}`
`122`	`124`	`size--;`
`123`	`125`	`}`