diff --git a/OnlineJudges/lydsy/1800.cpp b/OnlineJudges/lydsy/1800.cpp
new file mode 100644
index 0000000..76a304c
--- /dev/null
+++ b/OnlineJudges/lydsy/1800.cpp
@@ -0,0 +1,16 @@
+#include <cstdio>
+int len[25], ans;
+int main()
+{
+    int n;
+    scanf("%d", &n);
+    for (int i = 1; i <= n; i++) scanf("%d", &len[i]), len[i] += len[i - 1];
+    for (int i = 0 + 1; i <= n; i++)
+        for (int j = i + 1; j <= n; j++)
+            for (int k = j + 1; k <= n; k++)
+                for (int l = k + 1; l <= n; l++)
+                    if (len[j] - len[i] == len[l] - len[k] && len[n] - len[l] + len[i] == len[k] - len[j])
+                        ans++;
+    printf("%d", ans);
+    return 0;
+}
\ No newline at end of file
diff --git a/OnlineJudges/lydsy/2048.cpp b/OnlineJudges/lydsy/2048.cpp
new file mode 100644
index 0000000..74c204f
--- /dev/null
+++ b/OnlineJudges/lydsy/2048.cpp
@@ -0,0 +1,18 @@
+#include <cstdio>
+#include <cmath>
+const double eps = 1e-10, r = 0.5772156649;
+typedef long long ll;
+int main()
+{
+    double ans;
+    ll n, m;
+    ans = n = m = 0;
+    scanf("%lld%lld", &n, &m);
+    if (n <= 10000)
+        for (int i = 1; i <= n; i++)
+            ans += 0.5 / i;
+    else
+        ans = log(n + 1.0) + r, ans /= 2.0;
+    ans *= m;
+    printf("%d\n", (int)(ans - eps));
+}
\ No newline at end of file
diff --git a/OnlineJudges/lydsy/2120.cpp b/OnlineJudges/lydsy/2120.cpp
new file mode 100644
index 0000000..256397b
--- /dev/null
+++ b/OnlineJudges/lydsy/2120.cpp
@@ -0,0 +1,34 @@
+#include <cstdio>
+int a[10010], f[20010], map[1001001], idx;
+int main()
+{
+    int n, m;
+    scanf("%d%d", &n, &m);
+    for (int i = 1, x; i <= n; i++)
+    {
+        scanf("%d", &x);
+        if (!map[x])
+            map[x] = ++idx;
+        a[i] = map[x];
+    }
+    for (int i = 1, x, y; i <= m; i++)
+    {
+        char ch;
+        scanf("%s%d%d", &ch, &x, &y);
+        if (ch == 'R')
+        {
+            if (!map[y])
+                map[y] = ++idx;
+            a[x] = map[y];
+        }
+        else
+        {
+            int ans = 0;
+            for (int j = x; j <= y; j++)
+                if (f[a[j]] != i)
+                    ans++, f[a[j]] = i;
+            printf("%d\n", ans);
+        }
+    }
+    return 0;
+}
\ No newline at end of file
diff --git a/OnlineJudges/lydsy/2208.cpp b/OnlineJudges/lydsy/2208.cpp
new file mode 100644
index 0000000..b2eb7e6
--- /dev/null
+++ b/OnlineJudges/lydsy/2208.cpp
@@ -0,0 +1,39 @@
+#include <cstdio>
+#include <cstring>
+int n, ans;
+int next[2010 * 2010], to[2010 * 2010], head[2010], idx;
+char buf[2010];
+bool vis[2010];
+void addEdge(int f, int t)
+{
+    idx++;
+    next[idx] = head[f];
+    head[f] = idx;
+    to[idx] = t;
+}
+void dfs(int x)
+{
+    ans++;
+    vis[x] = true;
+    for (int cur = head[x]; cur; cur = next[cur])
+        if (!vis[to[cur]])
+            dfs(to[cur]);
+}
+int main()
+{
+    scanf("%d\n", &n);
+    for (int i = 0; i < n; i++)
+    {
+        fgets(buf, 2010, stdin);
+        for (int j = 0; j < n; j++)
+            if (buf[j] == '1')
+                addEdge(i + 1, j + 1);
+    }
+    for (int i = 1; i <= n; i++)
+    {
+        memset(vis, 0, sizeof(vis));
+        dfs(i);
+    }
+    printf("%d", ans);
+    return 0;
+}
\ No newline at end of file
diff --git a/OnlineJudges/lydsy/4195.cpp b/OnlineJudges/lydsy/4195.cpp
new file mode 100644
index 0000000..817eb06
--- /dev/null
+++ b/OnlineJudges/lydsy/4195.cpp
@@ -0,0 +1,36 @@
+#include <cstdio>
+#include <cctype>
+#include <map>
+inline void readInt(int &x)
+{
+    int ch = x = 0;
+    while (!isdigit(ch = getchar()));
+    for (; isdigit(ch); ch = getchar()) x = x * 10 + ch - '0';
+}
+int fa[2000010], I[1000010], J[1000010], E[1000010];
+int Find(int x) { return fa[x] == x ? x : fa[x] = Find(fa[x]); }
+int main()
+{
+    int t, n;
+    readInt(t);
+    while (t--)
+    {
+        readInt(n);
+        for (int i = 0; i < 200005; i++) fa[i] = i;
+        std::map<int, int> H;
+        for (int i = 0; i < n; i++)
+        {
+            readInt(I[i]), readInt(J[i]), readInt(E[i]);
+            if (H.find(I[i]) == H.end()) H[I[i]] = H.size() + 1;
+            if (H.find(J[i]) == H.end()) H[J[i]] = H.size() + 1;
+            if (H.find(E[i]) == H.end()) H[E[i]] = H.size() + 1;
+        }
+        for (int i = 0, x, y; i < n; i++)
+            if (E[i]) if ((x = Find(H[I[i]])) != (y = Find(H[J[i]]))) fa[x] = y;
+        bool flag = true;
+        for (int i = 0, x, y; i < n && flag; i++)
+            if (E[i] ^ 1) if ((x = Find(H[I[i]])) == (y = Find(H[J[i]]))) flag = false;
+        puts(flag ? "YES" : "NO");
+    }
+    return 0;
+}
\ No newline at end of file