Add Skip List (#3154)

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

@@ -0,0 +1,85 @@
+package com.thealgorithms.datastructures.lists;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.*;
+import java.util.stream.IntStream;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class SkipListTest {
+
+    @Test
+    void add() {
+        SkipList<String> skipList = new SkipList<>();
+        assertEquals(0, skipList.size());
+
+        skipList.add("value");
+
+        print(skipList);
+        assertEquals(1, skipList.size());
+    }
+
+    @Test
+    void get() {
+        SkipList<String> skipList = new SkipList<>();
+        skipList.add("value");
+
+        String actualValue = skipList.get(0);
+
+        print(skipList);
+        assertEquals("value", actualValue);
+    }
+
+    @Test
+    void contains() {
+        SkipList<String> skipList = createSkipList();
+        print(skipList);
+
+        boolean contains = skipList.contains("b");
+
+        assertTrue(contains);
+    }
+
+    @Test
+    void remove() {
+        SkipList<String> skipList = createSkipList();
+        int initialSize = skipList.size();
+        print(skipList);
+
+        skipList.remove("a");
+
+        print(skipList);
+        assertEquals(initialSize - 1, skipList.size());
+    }
+
+    @Test
+    void checkSortedOnLowestLayer() {
+        SkipList<String> skipList = new SkipList<>();
+        String[] values = {"d", "b", "a", "c"};
+        Arrays.stream(values).forEach(skipList::add);
+        print(skipList);
+
+        String[] actualOrder = IntStream.range(0, values.length)
+                .mapToObj(skipList::get)
+                .toArray(String[]::new);
+
+        assertArrayEquals(new String[]{"a", "b", "c", "d"}, actualOrder);
+    }
+
+    private SkipList<String> createSkipList() {
+        SkipList<String> skipList = new SkipList<>();
+        String[] values = {"a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k"};
+        Arrays.stream(values).forEach(skipList::add);
+        return skipList;
+    }
+
+    /**
+     * Print Skip List representation to console.
+     * Optional method not involved in testing process. Used only for visualisation purposes.
+     * @param skipList to print
+     */
+    private void print(SkipList<?> skipList) {
+        System.out.println(skipList);
+    }
+}