diff --git a/S2OJ/1651/1651.cpp b/S2OJ/1651/1651.cpp
new file mode 100644
index 00000000..1ed0a221
--- /dev/null
+++ b/S2OJ/1651/1651.cpp
@@ -0,0 +1,86 @@
+#include <iostream>
+#include <cmath>
+#include <vector>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 2e5 + 5;
+
+int n, q, k, b, l[N], id[N], w[N], cnt[505][505], t[N], sum, res;
+std::vector<int> pos[N], large;
+bool st[N];
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> n >> q >> k;
+
+    b = std::ceil(std::sqrt(n));
+
+    for (int i = 1; i <= n; i++) {
+        cin >> l[i];
+
+        pos[l[i]].emplace_back(i);
+
+        if (l[i] == l[i - 1]) w[l[i]]++;
+    }
+
+    for (int i = 1; i <= n; i++) {
+        if (pos[i].size() > b) {
+            large.emplace_back(i);
+            id[i] = large.size();
+        }
+    }
+
+    for (int i = 0; i < large.size(); i++) {
+        for (int x : pos[large[i]]) {
+            if (id[l[x - 1]]) {
+                cnt[i + 1][id[l[x - 1]]]++;
+            }
+
+            if (id[l[x + 1]]) {
+                cnt[i + 1][id[l[x + 1]]]++;
+            }
+        }
+    }
+
+    while (q--) {
+        int x;
+
+        cin >> x;
+
+        st[x] ^= 1;
+
+        int o = st[x] ? 1 : -1;
+
+        sum += o * pos[x].size();
+        res += o * w[x];
+
+        if (id[x]) {
+            for (int i = 1; i <= large.size(); i++) {
+                if (i == id[x]) continue;
+
+                if (st[large[i - 1]]) {
+                    res += o * cnt[id[x]][i];
+                }
+            }
+
+            res += o * t[x];
+        } else {
+            for (int y : pos[x]) {
+                if (st[l[y - 1]] && l[y - 1] != l[y]) res += o;
+                if (st[l[y + 1]] && l[y + 1] != l[y]) res += o;
+
+                if (id[l[y - 1]] && l[y - 1] != l[y]) t[l[y - 1]] += o;
+                if (id[l[y + 1]] && l[y + 1] != l[y]) t[l[y + 1]] += o;
+            }
+        }
+
+        cout << sum - res << endl;
+    }
+
+    return 0;
+}
diff --git a/S2OJ/1651/data/light1.ans b/S2OJ/1651/data/light1.ans
new file mode 100644
index 00000000..a4d53cbc
--- /dev/null
+++ b/S2OJ/1651/data/light1.ans
@@ -0,0 +1,3 @@
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+size 1884
diff --git a/S2OJ/1651/data/light1.in b/S2OJ/1651/data/light1.in
new file mode 100644
index 00000000..019a3652
--- /dev/null
+++ b/S2OJ/1651/data/light1.in
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new file mode 100644
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--- /dev/null
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