B - Who is missing?

https://atcoder.jp/contests/abc236/submissions/28730078

AtCoder/ABC236/B/B.cpp

@@ -0,0 +1,24 @@
+#include <iostream>
+#include <map>
+
+using std::cin;
+using std::cout;
+using std::endl;
+
+int n, a;
+std::map<int, int> map;
+
+int main() {
+    cin >> n;
+    for (int i = 1; i < n * 4; i++) {
+        cin >> a;
+        map[a]++;
+    }
+    for (int i = 1; i <= n; i++) {
+        if (map[i] == 3) {
+            cout << i << endl;
+            return 0;
+        }
+    }
+    return 0;
+}