This commit is related to BAEL-7722 (#16316)

This commit aims to add a test class "SymmetricSubstringMaxLengthUnitTest.java" to find length of longest symmetric substring.

core-java-modules/core-java-string-operations-8/src/test/java/com/baeldung/symmetricsubstringlength/SymmetricSubstringMaxLengthUnitTest.java

@@ -0,0 +1,77 @@
+package com.baeldung.symmetricsubstringlength;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class SymmetricSubstringMaxLengthUnitTest {
+    String input = "<><??>>";
+    int expected = 4;
+
+    @Test
+    public void givenString_whenUsingSymmetricSubstringExpansion_thenFindLongestSymmetricSubstring() {
+        int start = 0;
+        int mid = 0;
+        int last_gt = 0;
+        int end = 0;
+        int best = 0;
+
+        while (start < input.length()) {
+            int current = Math.min(mid - start, end - mid);
+            if (best < current) {
+                best = current;
+            }
+
+            if (end - mid == current && end < input.length()) {
+                if (input.charAt(end) == '?') {
+                    end++;
+                } else if (input.charAt(end) == '>') {
+                    end++;
+                    last_gt = end;
+                } else {
+                    end++;
+                    mid = end;
+                    start = Math.max(start, last_gt);
+                }
+            } else if (mid < input.length() && input.charAt(mid) == '?') {
+                mid++;
+            } else if (start < mid) {
+                start++;
+            } else {
+                start = Math.max(start, last_gt);
+                mid++;
+                end = Math.max(mid, end);
+            }
+        }
+        int result = 2 * best;
+
+        assertEquals(expected, result);
+    }
+
+    @Test
+    public void givenString_whenUsingBruteForce_thenFindLongestSymmetricSubstring() {
+        int max = 0;
+        for (int i = 0; i < input.length(); i++) {
+            for (int j = i + 1; j <= input.length(); j++) {
+                String t = input.substring(i, j);
+                if (t.length() % 2 == 0) {
+                    int k = 0, l = t.length() - 1;
+                    boolean isSym = true;
+                    while (k < l && isSym) {
+                        if (!(t.charAt(k) == '<' || t.charAt(k) == '?') && (t.charAt(l) == '>' || t.charAt(l) == '?')) {
+                            isSym = false;
+                        }
+                        k++;
+                        l--;
+                    }
+                    if (isSym) {
+                        max = Math.max(max, t.length());
+                    }
+                }
+            }
+        }
+
+        assertEquals(expected, max);
+    }
+
+}