diff --git a/khj20006/202511/10 BOJ P5 Y.md b/khj20006/202511/10 BOJ P5 Y.md
new file mode 100644
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+```cpp
+#include <bits/stdc++.h>
+using namespace std;
+
+int N;
+vector<vector<int>> v(100001);
+int d[100001]{}, dd[100001]{}, u[100001]{};
+int ans = 0;
+
+void dfs(int n, int p) {
+	int mx1 = 0, mx2 = 0, mx3 = 0;
+	for (int i : v[n]) if (i != p) {
+		dfs(i, n);
+		int res = d[i] + 1;
+		if (res > mx1) mx3 = mx2, mx2 = mx1, mx1 = res;
+		else if (res > mx2) mx3 = mx2, mx2 = res;
+		else if (res > mx3) mx3 = res;
+	}
+	d[n] = mx1, dd[n] = mx2;
+	if (mx3) ans = max(ans, mx1 + mx2 + mx3 + 1);
+}
+
+void dfs2(int n, int p) {
+	if (n != 1) u[n] = max(u[p], d[p] == d[n] + 1 ? dd[p] : d[p]) + 1;
+	if (u[n] && dd[n]) ans = max(ans, d[n] + dd[n] + u[n] + 1);
+	for (int i : v[n]) if (i != p) dfs2(i, n);
+}
+
+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);
+	dfs2(1, 0);
+	cout << ans;
+
+}
+```