diff --git a/AcWing/799/799.cpp b/AcWing/799/799.cpp
new file mode 100644
index 00000000..09b207a4
--- /dev/null
+++ b/AcWing/799/799.cpp
@@ -0,0 +1,21 @@
+#include <iostream>
+
+using std::cin;
+using std::cout;
+#define endl '\n'
+
+int n, a[100005], s[100005], ans;
+
+int main() {
+    cin >> n;
+    for (int i = 1; i <= n; i++) {
+        cin >> a[i];
+    }
+    for (int i = 1, j = 1; i <= n; i++) {
+        s[a[i]]++;
+        while (s[a[i]] > 1) s[a[j++]]--;
+        ans = std::max(ans, i - j + 1);
+    }
+    cout << ans << endl;
+    return 0;
+}