diff --git a/src/3/T179940_chat.cpp b/src/3/T179940_chat.cpp
new file mode 100644
index 0000000..9d0ce63
--- /dev/null
+++ b/src/3/T179940_chat.cpp
@@ -0,0 +1,114 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+typedef long long ll;
+
+// 定义一个极大值
+const ll INF = 1e18;
+
+// 线段树支持区间最小查询和点更新
+struct SegmentTree {
+    int n;
+    vector<ll> tree;
+
+    SegmentTree(int size) {
+        n = 1;
+        while(n < size) n <<= 1;
+        tree.assign(2*n, INF);
+    }
+
+    // 更新位置 pos (0-based) 的值为val
+    void update(int pos, ll val) {
+        pos += n;
+        tree[pos] = min(tree[pos], val);
+        while(pos > 1){
+            pos >>= 1;
+            tree[pos] = min(tree[2*pos], tree[2*pos+1]);
+        }
+    }
+
+    // 查询区间 [l, r) 的最小值
+    ll query(int l, int r) const {
+        ll res = INF;
+        int left = l + n;
+        int right = r + n;
+        while(left < right){
+            if(left & 1){
+                res = min(res, tree[left++]);
+            }
+            if(right & 1){
+                res = min(res, tree[--right]);
+            }
+            left >>= 1;
+            right >>= 1;
+        }
+        return res;
+    }
+};
+
+int main(){
+    ios::sync_with_stdio(false);
+    cin.tie(0);
+    int n;
+    ll m;
+    cin >> n >> m;
+    vector<ll> a(n);
+    for(auto &x:a) cin >> x;
+    
+    // 计算前缀和
+    vector<ll> s(n+1, 0);
+    for(int i=1;i<=n;i++) s[i] = s[i-1] + a[i-1];
+    
+    // 离散化前缀和
+    vector<ll> all_s = s;
+    // 还需要 x = s[i] - m
+    for(int i=0;i<=n;i++) all_s.push_back(s[i] - m);
+    sort(all_s.begin(), all_s.end());
+    all_s.erase(unique(all_s.begin(), all_s.end()), all_s.end());
+    
+    // 映射 s[j] 和 x 到离散化后的索引
+    auto get_idx = [&](ll val) -> int {
+        return lower_bound(all_s.begin(), all_s.end(), val) - all_s.begin();
+    };
+    
+    int size = all_s.size();
+    // 初始化两个线段树
+    SegmentTree st1(size); // 存 dp[j] - s[j]
+    SegmentTree st2(size); // 存 dp[j] + s[j]
+    
+    // 初始化 dp[0] = 0
+    // 插入 j=0 的情况
+    st1.update(get_idx(s[0]), 0 - s[0]);
+    st2.update(get_idx(s[0]), 0 + s[0]);
+    
+    // 初始化 dp array
+    // 可以不存储全 dp array，因为只需要 dp[j] 继续更新下去
+    vector<ll> dp(n+1, INF);
+    dp[0] = 0;
+    
+    for(int i=1;i<=n;i++){
+        ll x = s[i] - m;
+        // 找到 x 在 all_s 中的离散化索引
+        // 对于 st1，查询 s[j] <= x
+        // 找最大的 s[j] <= x，即 upper_bound(x) - 1
+        int pos1 = upper_bound(all_s.begin(), all_s.end(), x) - all_s.begin() - 1;
+        ll res1 = INF;
+        if(pos1 >= 0){
+            res1 = st1.query(0, pos1+1) + x;
+        }
+        // 对于 st2，查询 s[j] >= x
+        // 找最小的 s[j] >= x，即 lower_bound(x)
+        int pos2 = lower_bound(all_s.begin(), all_s.end(), x) - all_s.begin();
+        ll res2 = INF;
+        if(pos2 < size){
+            res2 = st2.query(pos2, size) - x;
+        }
+        // 取两者最小值作为 dp[i]
+        dp[i] = min(res1, res2);
+        // 更新线段树
+        st1.update(get_idx(s[i]), dp[i] - s[i]);
+        st2.update(get_idx(s[i]), dp[i] + s[i]);
+    }
+    
+    cout << dp[n] << "\n";
+}
\ No newline at end of file
diff --git a/src/3/T225686.cpp b/src/3/T225686.cpp
index 294989d..e9dcfc6 100644
--- a/src/3/T225686.cpp
+++ b/src/3/T225686.cpp
@@ -1,3 +1,75 @@
+#include <bits/stdc++.h>
+using namespace std;
+using ll = int64_t;
+
+ll n, m;
+
+pair<ll, ll> bfs(ll start, const vector<vector<ll>> &adj) {
+    vector<ll> dist(n + 1, -1);
+    queue<ll> q;
+    q.push(start);
+    dist[start] = 0;
+    ll far = start;
+
+    while (!q.empty()) {
+        ll u = q.front(); q.pop();
+        for (auto &v : adj[u]) {
+            if (dist[v] == -1) {
+                dist[v] = dist[u] + 1;
+                q.push(v);
+                if (dist[v] > dist[far]) {
+                    far = v;
+                }
+            }
+        }
+    }
+    return {far, dist[far]};
+}
+
 int main(){
+    ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+    cin >> n >> m;
+    vector<vector<ll>> adj(n + 1, vector<ll>());
+
+    for (ll i = 0; i < m; i++) {
+        ll u, v;
+        cin >> u >> v;
+        adj[u].push_back(v);
+        adj[v].push_back(u);
+    }
+
+    vector<bool> vis(n +1, false);
+
+    ll sum_d = 0;
+    ll k = 0;
     
+    for(ll u =1; u <=n; u++) {
+        if(!vis[u]){
+            k++;
+            auto [ff, fd1] = bfs(u, adj);
+            auto [sf, sd] = bfs(ff, adj);
+            sum_d += sd;
+
+            queue<ll> qvis;
+            qvis.push(u);
+            vis[u] = true;
+            while(!qvis.empty()){
+                ll node = qvis.front(); qvis.pop();
+                for(auto &v : adj[node]){
+                    if(!vis[v]){
+                        vis[v] = true;
+                        qvis.push(v);
+                    }
+                }
+            }
+        }
+    }
+
+    if(k == 1){
+        cout << sum_d + 1 << '\n';
+    }
+    else{
+        cout << sum_d + k << '\n';
+    }
 }
\ No newline at end of file
diff --git a/src/3/T225686_chat.cpp b/src/3/T225686_chat.cpp
new file mode 100644
index 0000000..4cfa1a6
--- /dev/null
+++ b/src/3/T225686_chat.cpp
@@ -0,0 +1,80 @@
+#include <bits/stdc++.h>
+using namespace std;
+using ll = int64_t;
+
+
+ll n, m;
+vector<vector<ll>> adj;
+vector<char> vis_marker; 
+
+
+pair<ll, ll> bfs(ll start){
+    deque<ll> q;
+    q.push_back(start);
+    ll farthest_node = start;
+    ll max_distance = 0;
+    vector<ll> dist(n + 1, -1);
+    dist[start] = 0;
+    vis_marker[start] = 1;
+
+    while(!q.empty()){
+        ll u = q.front();
+        q.pop_front();
+        for(auto &v : adj[u]){
+            if(!vis_marker[v]){
+                vis_marker[v] = 1;
+                dist[v] = dist[u] + 1;
+                q.push_back(v);
+                if(dist[v] > max_distance){
+                    max_distance = dist[v];
+                    farthest_node = v;
+                }
+            }
+        }
+    }
+    return {farthest_node, max_distance};
+}
+
+int main(){
+    ios::sync_with_stdio(false);
+    cin.tie(NULL);
+
+    cin >> n >> m;
+    adj.assign(n + 1, vector<ll>());
+    ll u, v;
+
+    for(ll i = 0; i < m; ++i){
+        cin >> u >> v;
+        adj[u].emplace_back(v);
+        adj[v].emplace_back(u);
+    }
+
+    
+    vis_marker.assign(n + 1, 0);
+
+    ll sum_d = 0;
+    ll k = 0;
+
+    for(ll u = 1; u <= n; ++u){
+        if(!vis_marker[u]){
+            k++;
+            
+            pair<ll, ll> first_bfs = bfs(u);
+            ll ff = first_bfs.first;
+
+            
+            pair<ll, ll> second_bfs = bfs(ff);
+            ll sd = second_bfs.second;
+
+            sum_d += sd;
+        }
+    }
+
+    
+    if(k == 1){
+        cout << sum_d + 1 << '\n';
+    }
+    else{
+        cout << sum_d + k << '\n';
+    }
+}
\ No newline at end of file
diff --git a/src/3/T585518_chat.cpp b/src/3/T585518_chat.cpp
new file mode 100644
index 0000000..13be19a
--- /dev/null
+++ b/src/3/T585518_chat.cpp
@@ -0,0 +1,60 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+// 定义无向图的邻接表表示
+vector<vector<int>> adj;
+
+// BFS函数，返回从起点开始的所有节点的距离
+vector<int> bfs(int start, int n) {
+    vector<int> dist(n + 1, -1); // 节点编号从1到n
+    queue<int> q;
+    q.push(start);
+    dist[start] = 0;
+    while (!q.empty()) {
+        int u = q.front(); q.pop();
+        for(auto &v : adj[u]) {
+            if(dist[v] == -1){
+                dist[v] = dist[u] + 1;
+                q.push(v);
+            }
+        }
+    }
+    return dist;
+}
+
+int main(){
+    ios::sync_with_stdio(false);
+    cin.tie(0);
+    
+    int n;
+    cin >> n;
+    adj.assign(n + 1, vector<int>());
+    for(int i=0;i<n-1;i++){
+        int u, v;
+        cin >> u >> v;
+        adj[u].push_back(v);
+        adj[v].push_back(u);
+    }
+
+    // 第一次 BFS，找出u
+    vector<int> dist1 = bfs(1, n);
+    int u = 1;
+    for(int i=1;i<=n;i++) {
+        if(dist1[i] > dist1[u]) u = i;
+    }
+
+    // 第二次 BFS，从u找出v，并记录d1
+    vector<int> d1 = bfs(u, n);
+    int v = u;
+    for(int i=1;i<=n;i++) {
+        if(d1[i] > d1[v]) v = i;
+    }
+
+    // 第三次 BFS，从v找出d2
+    vector<int> d2 = bfs(v, n);
+
+    // 输出每个节点的最远距离
+    for(int i=1;i<=n;i++) {
+        cout << max(d1[i], d2[i]) << (i==n ? '\n' : ' ');
+    }
+}
\ No newline at end of file
diff --git a/src/test.cpp b/src/test.cpp
new file mode 100644
index 0000000..394e26d
--- /dev/null
+++ b/src/test.cpp
@@ -0,0 +1,55 @@
+#include <iostream>
+#include <vector>
+#include <algorithm>
+using namespace std;
+
+struct TreeNode {
+    vector<int> children;
+};
+
+vector<TreeNode> tree;
+vector<int> enjoyment;
+vector<vector<long long>> dp;
+
+void dfs(int u, int parent) {
+    dp[u][0] = 0; 
+    dp[u][1] = enjoyment[u]; 
+    
+    for (int v : tree[u].children) {
+        if (v == parent) continue; 
+        dfs(v, u);
+        dp[u][0] += max(dp[v][0], dp[v][1]); 
+        dp[u][1] += dp[v][0]; 
+    }
+
+    for (int v : tree[u].children) {
+        if (v == parent) continue;
+        long long option = dp[u][1] - dp[v][0] + dp[v][1] - b; 
+        dp[u][1] = max(dp[u][1], option);
+    }
+}
+
+int main() {
+    int n, b;
+    cin >> n >> b;
+    enjoyment.resize(n + 1);
+    tree.resize(n + 1);
+    dp.resize(n + 1, vector<long long>(2));
+    
+    for (int i = 1; i <= n; ++i) {
+        cin >> enjoyment[i];
+    }
+    
+    for (int i = 0; i < n - 1; ++i) {
+        int x, y;
+        cin >> x >> y;
+        tree[x].children.push_back(y);
+        tree[y].children.push_back(x);
+    }
+
+    dfs(1, -1); 
+    long long result = max(0LL, max(dp[1][0], dp[1][1])); 
+    cout << result << endl;
+
+    return 0;
+}
\ No newline at end of file