To add Suffix array algorithm implementation (#6530)

* adding Suffix Array Implementation

* adding test for Suffix Array

* patch: lint fix in SuffixArray.java

* patch: lint fix in SuffixArrayTest.java

* fix: private constructor update

* patch: lint fix in suffix array impl

* fix: lint issue line blankspace removal

* fix: checkstype issue

* fix: lint format issue

* Update DIRECTORY.md

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

@@ -0,0 +1,44 @@
+package com.thealgorithms.strings;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+
+import org.junit.jupiter.api.Test;
+
+class SuffixArrayTest {
+
+    @Test
+    void testEmptyString() {
+        int[] result = SuffixArray.buildSuffixArray("");
+        assertArrayEquals(new int[] {}, result, "Empty string should return empty suffix array");
+    }
+
+    @Test
+    void testSingleCharacter() {
+        int[] result = SuffixArray.buildSuffixArray("a");
+        assertArrayEquals(new int[] {0}, result, "Single char string should return [0]");
+    }
+
+    @Test
+    void testDistinctCharacters() {
+        int[] result = SuffixArray.buildSuffixArray("abc");
+        assertArrayEquals(new int[] {0, 1, 2}, result, "Suffixes already in order for distinct chars");
+    }
+
+    @Test
+    void testBananaExample() {
+        int[] result = SuffixArray.buildSuffixArray("banana");
+        assertArrayEquals(new int[] {5, 3, 1, 0, 4, 2}, result, "Suffix array of 'banana' should be [5,3,1,0,4,2]");
+    }
+
+    @Test
+    void testStringWithDuplicates() {
+        int[] result = SuffixArray.buildSuffixArray("aaaa");
+        assertArrayEquals(new int[] {3, 2, 1, 0}, result, "Suffix array should be descending indices for 'aaaa'");
+    }
+
+    @Test
+    void testRandomString() {
+        int[] result = SuffixArray.buildSuffixArray("mississippi");
+        assertArrayEquals(new int[] {10, 7, 4, 1, 0, 9, 8, 6, 3, 5, 2}, result, "Suffix array for 'mississippi' should match expected");
+    }
+}