Add longest substring (#6007)

src/test/java/com/thealgorithms/slidingwindow/LongestSubstringWithoutRepeatingCharactersTest.java

@@ -0,0 +1,23 @@
+package com.thealgorithms.slidingwindow;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Unit tests for the LongestSubstringWithoutRepeatingCharacters class.
+ *
+ * @author  (https://github.com/Chiefpatwal)
+ */
+public class LongestSubstringWithoutRepeatingCharactersTest {
+
+    @Test
+    public void testLengthOfLongestSubstring() {
+        // Test cases for the lengthOfLongestSubstring method
+        assertEquals(3, LongestSubstringWithoutRepeatingCharacters.lengthOfLongestSubstring("abcabcbb"));
+        assertEquals(1, LongestSubstringWithoutRepeatingCharacters.lengthOfLongestSubstring("bbbbb"));
+        assertEquals(3, LongestSubstringWithoutRepeatingCharacters.lengthOfLongestSubstring("pwwkew"));
+        assertEquals(0, LongestSubstringWithoutRepeatingCharacters.lengthOfLongestSubstring(""));
+        assertEquals(5, LongestSubstringWithoutRepeatingCharacters.lengthOfLongestSubstring("abcde"));
+    }
+}