fix(lists): leave SkipList size unchanged when removing a missing element (#7666)

Author: Rishavbhattarai

src/main/java/com/thealgorithms/datastructures/lists/SkipList.java

@@ -121,6 +121,9 @@ public void remove(E e) {
                 current = next;
             }
         }
+        if (layer < 0) {
+            return; // not found, nothing to unlink
+        }
         for (int i = 0; i <= layer; i++) {
             current.previous(i).setNext(i, current.next(i));
             if (current.next(i) != null) {

src/test/java/com/thealgorithms/datastructures/lists/SkipListTest.java

@@ -106,6 +106,20 @@ void testClearViaRemovals() {
         assertEquals(0, skipList.size());
     }
 
+    @Test
+    @DisplayName("Removing a missing element leaves the list unchanged")
+    void testRemoveMissing() {
+        skipList.remove("missing");
+        assertEquals(0, skipList.size());
+
+        skipList.add("a");
+        skipList.add("c");
+        skipList.remove("b");
+        assertEquals(2, skipList.size());
+        assertEquals("a", skipList.get(0));
+        assertEquals("c", skipList.get(1));
+    }
+
     private SkipList<String> createSkipList() {
         SkipList<String> s = new SkipList<>();
         String[] values = {"a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"};