refactor: RemoveDuplicateFromString (#5387)

src/test/java/com/thealgorithms/others/RemoveDuplicateFromStringTest.java

@@ -0,0 +1,44 @@
+package com.thealgorithms.others;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+import org.junit.jupiter.api.Test;
+
+class RemoveDuplicateFromStringTest {
+
+    @Test
+    void testEmptyString() {
+        assertEquals("", RemoveDuplicateFromString.removeDuplicate(""));
+    }
+
+    @Test
+    void testNullString() {
+        assertNull(RemoveDuplicateFromString.removeDuplicate(null));
+    }
+
+    @Test
+    void testSingleCharacterString() {
+        assertEquals("a", RemoveDuplicateFromString.removeDuplicate("a"));
+    }
+
+    @Test
+    void testStringWithNoDuplicates() {
+        assertEquals("abc", RemoveDuplicateFromString.removeDuplicate("abc"));
+    }
+
+    @Test
+    void testStringWithDuplicates() {
+        assertEquals("abcd", RemoveDuplicateFromString.removeDuplicate("aabbbccccddddd"));
+    }
+
+    @Test
+    void testStringWithAllSameCharacters() {
+        assertEquals("a", RemoveDuplicateFromString.removeDuplicate("aaaaa"));
+    }
+
+    @Test
+    void testStringWithMixedCase() {
+        assertEquals("abAB", RemoveDuplicateFromString.removeDuplicate("aabABAB"));
+    }
+}