[20251109] BOJ / G4 / 대전 도시철도 2호선 / 권혁준

khj20006/202511/09 BOJ G4 대전 도시철도 2호선.md

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+```cpp
+#include <bits/stdc++.h>
+using namespace std;
+
+int N;
+vector<vector<int>> v(100001);
+vector<int> arr;
+bitset<100001> vis;
+
+void dfs(int n, int p) {
+    if(n == N) vis[n] = 1;
+    for(int i:v[n]) if(i != p) {
+        dfs(i,n);
+        if(vis[i]) vis[n] = 1;
+    }
+}
+
+int main() {
+    cin.tie(0)->sync_with_stdio(0);
+
+    cin>>N;
+    for(int i=1,a,b;i<N;i++) {
+        cin>>a>>b;
+        v[a].push_back(b);
+        v[b].push_back(a);
+    }
+
+    dfs(1,0);
+    for(int i=1;i<=N;i++) if(!vis[i]) {
+        int cnt = 0;
+        queue<int> q;
+        vis[i] = 1;
+        q.push(i);
+        while(!q.empty()) {
+            int n = q.front(); q.pop();
+            cnt++;
+            for(int j:v[n]) if(!vis[j]) {
+                vis[j] = 1;
+                q.push(j);
+            }
+        }
+        arr.push_back(cnt);
+    }
+    long long sum = 0, ans = 0;
+    for(int i:arr) {
+        ans += sum * i;
+        sum += i;
+    }
+    cout<<ans;
+
+}
+```