Fix MoveHash: correct null handling and add comprehensive test coverage

Author: ShyamChavda005

src/main/java/com/thealgorithms/twopointer/MoveHash.java

@@ -15,6 +15,7 @@
 public class MoveHash {
 
 
+
     /** default constructor  */
     public MoveHash() {}
 
@@ -27,6 +28,11 @@ public MoveHash() {}
      */
 
     public static String movehashtoend(String s) {
+        /** return null if inputed string is null */
+        if (s == null) {
+            return null;
+        }
+
         /** converts string into character array
           for example.,
                string = roman,
@@ -35,11 +41,6 @@ public static String movehashtoend(String s) {
 
         char[] c = s.toCharArray();
 
-        /** return null if inputed string is null */
-        if (s == null) {
-            return "null";
-        }
-
         /** j works like a tracker the hash position */
         int j = 0;
 

src/test/java/com/thealgorithms/twopointer/MoveHashTest.java

@@ -1,6 +1,7 @@
 package com.thealgorithms.twopointer;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
 
 import org.junit.jupiter.api.Test;
 
@@ -18,6 +19,44 @@ void testEmpty() {
 
     @Test
     void testNull() {
-        assertEquals(null, MoveHash.movehashtoend(null));
+        assertNull(MoveHash.movehashtoend(null));
+    }
+
+    @Test
+    void testNoHash() {
+        assertEquals("hello", MoveHash.movehashtoend("hello"));
+    }
+
+    @Test
+    void testAllHash() {
+        assertEquals("#####", MoveHash.movehashtoend("#####"));
+    }
+
+    @Test
+    void testHashAtEnd() {
+        assertEquals("hello##", MoveHash.movehashtoend("hello##"));
+    }
+
+    @Test
+    void testHashAtStart() {
+        assertEquals("hello#####", MoveHash.movehashtoend("######hello"));
+    }
+
+    @Test
+    void testSingleCharacter() {
+        assertEquals("#", MoveHash.movehashtoend("#"));
+    }
+
+    @Test
+    void testSingleCharacterNoHash() {
+        assertEquals("a", MoveHash.movehashtoend("a"));
+    }
+
+    @Test
+    void testSwapFunction() {
+        char[] arr = {'a', 'b', 'c'};
+        MoveHash.swap(0, 2, arr);
+        assertEquals('c', arr[0]);
+        assertEquals('a', arr[2]);
     }
 }