feat: add RansomNote algorithm with tests

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

@@ -0,0 +1,29 @@
+package com.thealgorithms.strings;
+
+/**
+ * Determines if a ransom note can be constructed using letters from a magazine.
+ * Each letter in the magazine can only be used once.
+ *
+ * <p>Example: ransomNote = "aa", magazine = "aab" -> true
+ *
+ * <p>Time complexity: O(n), Space complexity: O(1) (fixed 26-char alphabet)
+ *
+ * @see <a href="https://leetcode.com/problems/ransom-note/">LeetCode #383</a>
+ */
+public final class RansomNote {
+    private RansomNote() {
+    }
+
+    public static boolean canConstruct(String ransomNote, String magazine) {
+        int[] count = new int[26];
+        for (char c : magazine.toCharArray()) {
+            count[c - 'a']++;
+        }
+        for (char c : ransomNote.toCharArray()) {
+            if (--count[c - 'a'] < 0) {
+                return false;
+            }
+        }
+        return true;
+    }
+}

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

@@ -0,0 +1,34 @@
+package com.thealgorithms.strings;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
+public class RansomNoteTest {
+
+    @Test
+    public void testCanConstruct() {
+        assertTrue(RansomNote.canConstruct("aa", "aab"));
+    }
+
+    @Test
+    public void testCannotConstruct() {
+        assertFalse(RansomNote.canConstruct("aa", "ab"));
+    }
+
+    @Test
+    public void testEmptyRansomNote() {
+        assertTrue(RansomNote.canConstruct("", "abc"));
+    }
+
+    @Test
+    public void testEmptyMagazine() {
+        assertFalse(RansomNote.canConstruct("a", ""));
+    }
+
+    @Test
+    public void testExactMatch() {
+        assertTrue(RansomNote.canConstruct("abc", "abc"));
+    }
+}