Enhance docs, add tests in LinearProbingHashMap (#5977)

src/test/java/com/thealgorithms/datastructures/hashmap/hashing/LinearProbingHashMapTest.java

@@ -1,8 +1,91 @@
 package com.thealgorithms.datastructures.hashmap.hashing;
 
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
 class LinearProbingHashMapTest extends MapTest {
+
     @Override
     <Key extends Comparable<Key>, Value> Map<Key, Value> getMap() {
         return new LinearProbingHashMap<>();
     }
+
+    @Test
+    void putNullKey() {
+        Map<Integer, String> map = getMap();
+        assertFalse(map.put(null, "value"), "Putting a null key should return false");
+    }
+
+    @Test
+    void putDuplicateKeys() {
+        Map<Integer, String> map = getMap();
+        map.put(1, "one");
+        map.put(1, "uno");
+        assertEquals("uno", map.get(1), "Value should be updated to 'uno'");
+    }
+
+    @Test
+    void putResizeTest() {
+        Map<Integer, String> map = getMap();
+        for (int i = 0; i < 20; i++) {
+            map.put(i, String.valueOf(i));
+        }
+        assertEquals(20, map.size(), "Map size should be 20 after inserting 20 elements");
+    }
+
+    @Test
+    void deleteNonExistentKey() {
+        Map<Integer, String> map = getMap();
+        assertFalse(map.delete(999), "Deleting a non-existent key should return false");
+    }
+
+    @Test
+    void deleteAndReinsert() {
+        Map<Integer, String> map = getMap();
+        map.put(1, "one");
+        map.delete(1);
+        assertFalse(map.contains(1), "Map should not contain the deleted key");
+        map.put(1, "one again");
+        assertTrue(map.contains(1), "Map should contain the key after reinsertion");
+    }
+
+    @Test
+    void resizeDown() {
+        Map<Integer, String> map = getMap();
+        for (int i = 0; i < 16; i++) {
+            map.put(i, String.valueOf(i));
+        }
+        for (int i = 0; i < 12; i++) {
+            map.delete(i);
+        }
+        assertEquals(4, map.size(), "Map size should be 4 after deleting 12 elements");
+    }
+
+    @Test
+    void keysOrderTest() {
+        Map<Integer, String> map = getMap();
+        for (int i = 10; i > 0; i--) {
+            map.put(i, String.valueOf(i));
+        }
+        int expectedKey = 1;
+        for (Integer key : map.keys()) {
+            assertEquals(expectedKey++, key, "Keys should be in sorted order");
+        }
+    }
+
+    @Test
+    void stressTest() {
+        Map<Integer, String> map = getMap();
+        for (int i = 0; i < 1000; i++) {
+            map.put(i, String.valueOf(i));
+            assertEquals(i + 1, map.size(), "Size should match number of inserted elements");
+        }
+        for (int i = 0; i < 500; i++) {
+            map.delete(i);
+            assertEquals(1000 - (i + 1), map.size(), "Size should decrease correctly");
+        }
+    }
 }