Add solution for "Destination City" with unit tests.

src/main/java/com/leetcode/easy/DestinationCity1436.java

@@ -0,0 +1,32 @@
+// Tags: Array, Hash Set
+package com.leetcode.easy;
+
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+
+public class DestinationCity1436 {
+    /*
+    Given n as the length of paths,
+
+    Time complexity: O(n)
+    We first iterate over paths to populate hasOutgoing, this costs O(n).
+    Next, we iterate over paths again to find the answer, checking at each step whether candidate is in the hash set, which takes O(1). Thus the iteration costs O(n).
+
+    Space complexity: O(n)
+    hasOutgoing will grow to a size of O(n).
+     */
+    public String destCity(List<List<String>> paths) {
+        Set<String> hasOutgoing = new HashSet<>();
+        for (List<String> path : paths) {
+            hasOutgoing.add(path.get(0));
+        }
+        for (List<String> path : paths) {
+            String candidate = path.get(1);
+            if (!hasOutgoing.contains(candidate)) {
+                return candidate;
+            }
+        }
+        return "";
+    }
+}

src/test/java/com/leetcode/easy/DestinationCity1436Test.java

@@ -0,0 +1,51 @@
+package com.leetcode.easy;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class DestinationCity1436Test {
+    private DestinationCity1436 solution;
+
+    @BeforeEach
+    void setUp() {
+        solution = new DestinationCity1436();
+    }
+
+    @Test
+    void testDestCity_Example1() {
+        List<List<String>> paths = List.of(
+                List.of("London", "New York"),
+                List.of("New York", "Lima"),
+                List.of("Lima", "Sao Paulo")
+        );
+        String expected = "Sao Paulo";
+        String result = solution.destCity(paths);
+        assertEquals(expected, result);
+    }
+
+    @Test
+    void testDestCity_Example2() {
+        List<List<String>> paths = List.of(
+                List.of("B", "C"),
+                List.of("D", "B"),
+                List.of("C", "A")
+        );
+        String expected = "A";
+        String result = solution.destCity(paths);
+        assertEquals(expected, result);
+    }
+
+    @Test
+    void testDestCity_Example3() {
+        List<List<String>> paths = List.of(
+                List.of("A", "Z")
+        );
+        String expected = "Z";
+        String result = solution.destCity(paths);
+        assertEquals(expected, result);
+    }
+}