From d4e1ac9b56cd76f3794546f7598df92b89f5996e Mon Sep 17 00:00:00 2001
From: Peter Chen <sanpeter992000@gmail.com>
Date: Mon, 22 Sep 2025 16:35:48 +0800
Subject: [PATCH] Add solution for "Destination City" with unit tests.

---
 src/DestinationCity1436.java                  | 31 -----------
 .../leetcode/easy/DestinationCity1436.java    | 32 ++++++++++++
 .../easy/DestinationCity1436Test.java         | 51 +++++++++++++++++++
 3 files changed, 83 insertions(+), 31 deletions(-)
 delete mode 100644 src/DestinationCity1436.java
 create mode 100644 src/main/java/com/leetcode/easy/DestinationCity1436.java
 create mode 100644 src/test/java/com/leetcode/easy/DestinationCity1436Test.java

diff --git a/src/DestinationCity1436.java b/src/DestinationCity1436.java
deleted file mode 100644
index b159a27..0000000
--- a/src/DestinationCity1436.java
+++ /dev/null
@@ -1,31 +0,0 @@
-import java.util.ArrayList;
-import java.util.HashMap;
-import java.util.List;
-import java.util.Map;
-
-public class DestinationCity1436 {
-    public String destCity(List<List<String>> paths) {
-        Map<String, List<String>> map = new HashMap<>();
-        for (List<String> path : paths) {
-            String src = path.get(0);
-            String dst = path.get(1);
-            if (!map.containsKey(src)) {
-                List<String> dsts = new ArrayList<>();
-                dsts.add(dst);
-                map.put(src, dsts);
-            } else {
-                List<String> dsts = map.get(src);
-                dsts.add(dst);
-                map.put(src, dsts);
-            }
-        }
-        for (Map.Entry<String, List<String>> entry : map.entrySet()) {
-            for (String dst : entry.getValue()) {
-                if (!map.containsKey(dst)) {
-                    return dst;
-                }
-            }
-        }
-        return "";
-    }
-}
diff --git a/src/main/java/com/leetcode/easy/DestinationCity1436.java b/src/main/java/com/leetcode/easy/DestinationCity1436.java
new file mode 100644
index 0000000..b30294f
--- /dev/null
+++ b/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 "";
+    }
+}
diff --git a/src/test/java/com/leetcode/easy/DestinationCity1436Test.java b/src/test/java/com/leetcode/easy/DestinationCity1436Test.java
new file mode 100644
index 0000000..8e21994
--- /dev/null
+++ b/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);
+    }
+}