feat(cpp): add thtbangc2025_bai1

cpp/thtbangc2025_bai1.cpp

@@ -0,0 +1,25 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+int main() {
+    ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    int N;
+    cin >> N;
+    map<string, int> count;
+    for (int i = 0; i < 2 * N - 1; ++i) {
+        string s;
+        cin >> s;
+        count[s]++;
+    }
+
+    for (auto &p: count) {
+        if (p.second % 2 == 1) {
+            cout << p.first << "\n";
+            break;
+        }
+    }
+
+    return 0;
+}