feat: add FindTheDifference algorithm with tests

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

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+package com.thealgorithms.strings;
+
+/**
+ * Finds the extra character added to string t, which is string s with one
+ * additional random character shuffled in.
+ *
+ * <p>Example: s = "abcd", t = "abcde" -> 'e'
+ *
+ * <p>Uses XOR to cancel out matching characters in O(n) time and O(1) space.
+ *
+ * @see <a href="https://leetcode.com/problems/find-the-difference/">LeetCode #389</a>
+ */
+public final class FindTheDifference {
+    private FindTheDifference() {
+    }
+
+    public static char findTheDifference(String s, String t) {
+        char result = 0;
+        for (char c : s.toCharArray()) {
+            result ^= c;
+        }
+        for (char c : t.toCharArray()) {
+            result ^= c;
+        }
+        return result;
+    }
+}

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

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+package com.thealgorithms.strings;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class FindTheDifferenceTest {
+
+    @Test
+    public void testExample() {
+        assertEquals('e', FindTheDifference.findTheDifference("abcd", "abcde"));
+    }
+
+    @Test
+    public void testEmptyS() {
+        assertEquals('a', FindTheDifference.findTheDifference("", "a"));
+    }
+
+    @Test
+    public void testSameCharAdded() {
+        assertEquals('a', FindTheDifference.findTheDifference("a", "aa"));
+    }
+
+    @Test
+    public void testShuffled() {
+        assertEquals('y', FindTheDifference.findTheDifference("ae", "eay"));
+    }
+}