feat: Add CelebrityFinder new algorithm with Junit tests

DIRECTORY.md

@@ -607,6 +607,7 @@
             * [WiggleSort](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/sorts/WiggleSort.java)
           * stacks
             * [BalancedBrackets](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/BalancedBrackets.java)
+            * [CelebrityFinder](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/CelebrityFinder.java)
             * [DecimalToAnyUsingStack](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/DecimalToAnyUsingStack.java)
             * [DuplicateBrackets](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/DuplicateBrackets.java)
             * [InfixToPostfix](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/InfixToPostfix.java)
@@ -1163,6 +1164,7 @@
             * [WiggleSortTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/sorts/WiggleSortTest.java)
           * stacks
             * [BalancedBracketsTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/BalancedBracketsTest.java)
+            * [CelebrityFinderTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/CelebrityFinderTest.java)
             * [DecimalToAnyUsingStackTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/DecimalToAnyUsingStackTest.java)
             * [DuplicateBracketsTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/DuplicateBracketsTest.java)
             * [InfixToPostfixTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/InfixToPostfixTest.java)

src/main/java/com/thealgorithms/stacks/CelebrityFinder.java

@@ -0,0 +1,52 @@
+package com.thealgorithms.stacks;
+
+import java.util.Stack;
+
+/**
+ * Solves the celebrity problem using a stack-based algorithm.
+ *
+ * <p>Celebrity is someone known by everyone but doesn't know anyone else.
+ * <p>Applications: Graph theory and social network analysis.
+ *
+ * @author Hardvan
+ */
+public final class CelebrityFinder {
+    private CelebrityFinder() {
+    }
+
+    /**
+     * Finds the celebrity in the given party matrix using a stack-based algorithm.
+     *
+     * @param party A 2D matrix where party[i][j] is 1 if i knows j, otherwise 0.
+     * @return The index of the celebrity, or -1 if there is no celebrity.
+     */
+    public static int findCelebrity(int[][] party) {
+
+        // Push all people onto the stack
+        Stack<Integer> stack = new Stack<>();
+        for (int i = 0; i < party.length; i++) {
+            stack.push(i);
+        }
+
+        // Find the potential celebrity by comparing pairs
+        while (stack.size() > 1) {
+            int person1 = stack.pop();
+            int person2 = stack.pop();
+
+            if (party[person1][person2] == 1) {
+                stack.push(person2); // person1 knows person2, so person2 might be the celebrity
+            } else {
+                stack.push(person1); // person1 doesn't know person2, so person1 might be the celebrity
+            }
+        }
+
+        // Verify the candidate
+        int candidate = stack.pop();
+        for (int i = 0; i < party.length; i++) {
+            if (i != candidate && (party[candidate][i] == 1 || party[i][candidate] == 0)) {
+                return -1;
+            }
+        }
+        return candidate;
+    }
+}

src/test/java/com/thealgorithms/stacks/CelebrityFinderTest.java

@@ -0,0 +1,41 @@
+package com.thealgorithms.stacks;
+
+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;
+
+public class CelebrityFinderTest {
+
+    @ParameterizedTest
+    @MethodSource("providePartyMatrices")
+    public void testCelebrityFinder(int[][] party, int expected) {
+        assertEquals(expected, CelebrityFinder.findCelebrity(party));
+    }
+
+    private static Stream<Arguments> providePartyMatrices() {
+        return Stream.of(
+            // Test case 1: Celebrity exists
+            Arguments.of(new int[][] {{0, 1, 1}, {0, 0, 1}, {0, 0, 0}}, 2),
+
+            // Test case 2: No celebrity
+            Arguments.of(new int[][] {{0, 1, 0}, {1, 0, 1}, {1, 1, 0}}, -1),
+
+            // Test case 3: Everyone knows each other, no celebrity
+            Arguments.of(new int[][] {{0, 1, 1}, {1, 0, 1}, {1, 1, 0}}, -1),
+
+            // Test case 4: Single person, they are trivially a celebrity
+            Arguments.of(new int[][] {{0}}, 0),
+
+            // Test case 5: All know the last person, and they know no one
+            Arguments.of(new int[][] {{0, 1, 1, 1}, {0, 0, 1, 1}, {0, 0, 0, 1}, {0, 0, 0, 0}}, 3),
+
+            // Test case 6: Larger party with no celebrity
+            Arguments.of(new int[][] {{0, 1, 1, 0}, {1, 0, 0, 1}, {0, 1, 0, 1}, {1, 1, 1, 0}}, -1),
+
+            // Test case 7: Celebrity at the start of the matrix
+            Arguments.of(new int[][] {{0, 0, 0}, {1, 0, 1}, {1, 1, 0}}, 0));
+    }
+}