Add itinerary tickets code

src/main/java/com/thealgorithms/datastructures/hashmap/hashing/ItineraryTickets.java

@@ -0,0 +1,29 @@
+package com.thealgorithms.datastructures.hashmap.hashing;
+
+import java.util.HashMap;
+
+/**
+ * This class represents an itinerary of tickets. Given a list of tickets, it finds the
+ * complete itinerary.
+ */
+public class ItineraryTickets {
+
+    /**
+     * This method finds the starting point of the itinerary.
+     *
+     * @param tickets A map of tickets where the key is the departure point, and the value is the destination.
+     * @return The starting point of the itinerary.
+     */
+    public static String getStart(HashMap<String, String> tickets) {
+        HashMap<String, String> revTickets = new HashMap<>();
+        for (String key : tickets.keySet()) {
+            revTickets.put(tickets.get(key), key);
+        }
+        for (String key : tickets.keySet()) {
+            if (!revTickets.containsKey(key)) {
+                return key; // Starting point
+            }
+        }
+        return null;
+    }
+}

src/test/java/com/thealgorithms/datastructures/hashmap/hashing/ItineraryTicketsTest.java

@@ -0,0 +1,44 @@
+package com.thealgorithms.datastructures.hashmap.hashing;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.util.HashMap;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+public class ItineraryTicketsTest {
+
+    private HashMap<String, String> tickets;
+
+    @BeforeEach
+    public void setUp() {
+        tickets = new HashMap<>();
+        tickets.put("Chennai", "Bengaluru");
+        tickets.put("Mumbai", "Delhi");
+        tickets.put("Goa", "Chennai");
+        tickets.put("Delhi", "Goa");
+    }
+
+    @Test
+    public void testGetStart() {
+        String start = ItineraryTickets.getStart(tickets);
+        assertEquals("Mumbai", start);
+    }
+
+    @Test
+    public void testGetStartEmptyInput() {
+        HashMap<String, String> emptyTickets = new HashMap<>();
+        String start = ItineraryTickets.getStart(emptyTickets);
+        assertNull(start);
+    }
+
+    @Test
+    public void testGetStartCircularItinerary() {
+
+        tickets.put("Bengaluru", "Mumbai");
+        String start = ItineraryTickets.getStart(tickets);
+        assertNull(start); 
+    }
+}