test: LongestPalindromicSubstring (#5402)

* LongestPalindromicSubstring

* fix Rule:CollapsibleIfStatements

---------

Co-authored-by: alxkm <alx@alx.com>

src/main/java/com/thealgorithms/strings/LongestPalindromicSubstring.java

@@ -1,44 +1,31 @@
 package com.thealgorithms.strings;
 
-// Longest Palindromic Substring
-import java.util.Scanner;
-
 final class LongestPalindromicSubstring {
     private LongestPalindromicSubstring() {
     }
 
-    public static void main(String[] args) {
-        Solution s = new Solution();
-        String str = "";
-        Scanner sc = new Scanner(System.in);
-        System.out.print("Enter the string: ");
-        str = sc.nextLine();
-        System.out.println("Longest substring is : " + s.longestPalindrome(str));
-        sc.close();
-    }
-}
-
-class Solution {
-
-    public String longestPalindrome(String s) {
-        if (s == null || s.length() == 0) {
+    /**
+     * Finds the longest palindromic substring in the given string.
+     *
+     * @param s the input string
+     * @return the longest palindromic substring
+     */
+    public static String longestPalindrome(String s) {
+        if (s == null || s.isEmpty()) {
             return "";
         }
-        int n = s.length();
         String maxStr = "";
-        for (int i = 0; i < n; ++i) {
-            for (int j = i; j < n; ++j) {
-                if (isValid(s, i, j)) {
-                    if (j - i + 1 > maxStr.length()) { // update maxStr
-                        maxStr = s.substring(i, j + 1);
-                    }
+        for (int i = 0; i < s.length(); ++i) {
+            for (int j = i; j < s.length(); ++j) {
+                if (isValid(s, i, j) && (j - i + 1 > maxStr.length())) {
+                    maxStr = s.substring(i, j + 1);
                 }
             }
         }
         return maxStr;
     }
 
-    private boolean isValid(String s, int lo, int hi) {
+    private static boolean isValid(String s, int lo, int hi) {
         int n = hi - lo + 1;
         for (int i = 0; i < n / 2; ++i) {
             if (s.charAt(lo + i) != s.charAt(hi - i)) {

src/test/java/com/thealgorithms/strings/LongestPalindromicSubstringTest.java

@@ -0,0 +1,21 @@
+package com.thealgorithms.strings;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.stream.Stream;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class LongestPalindromicSubstringTest {
+
+    @ParameterizedTest
+    @MethodSource("provideTestCasesForLongestPalindrome")
+    void testLongestPalindrome(String input, String expected) {
+        assertEquals(expected, LongestPalindromicSubstring.longestPalindrome(input));
+    }
+
+    private static Stream<Arguments> provideTestCasesForLongestPalindrome() {
+        return Stream.of(Arguments.of("babad", "bab"), Arguments.of("cbbd", "bb"), Arguments.of("a", "a"), Arguments.of("", ""), Arguments.of("abc", "a"), Arguments.of(null, ""), Arguments.of("aaaaa", "aaaaa"));
+    }
+}