From 401d633dbe7daff093d33ac076355716d0c65cbe Mon Sep 17 00:00:00 2001
From: Ryo Suzuki <reputeless+github@gmail.com>
Date: Mon, 9 Jan 2023 13:48:13 +0900
Subject: [PATCH] update

---
 39.md     | 66 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 README.md |  2 +-
 2 files changed, 67 insertions(+), 1 deletion(-)
 create mode 100644 39.md

diff --git a/39.md b/39.md
new file mode 100644
index 0000000..aa6930c
--- /dev/null
+++ b/39.md
@@ -0,0 +1,66 @@
+# 039 - Tree Distance (★5)
+
+## 解答
+
+```cpp
+#include <iostream>
+#include <vector>
+#include <algorithm>
+
+void DFS(const std::vector<std::vector<int>>& graph, int from, int prev, std::vector<int>& dp)
+{
+	dp[from] = 1;
+
+	for (const auto& to : graph[from])
+	{
+		if (to != prev)
+		{
+			DFS(graph, to, from, dp);
+
+			// 帰りがけ順で dp を更新
+			dp[from] += dp[to];
+		}
+	}
+}
+
+int main()
+{
+	int N;
+	std::cin >> N;
+
+	std::vector<int> A(N - 1), B(N - 1);
+	std::vector<std::vector<int>> graph(N);
+	for (int i = 0; i < (N - 1); ++i)
+	{
+		std::cin >> A[i] >> B[i];
+		--A[i]; --B[i];
+		graph[A[i]].push_back(B[i]);
+		graph[B[i]].push_back(A[i]);
+	}
+
+	std::vector<bool> visited(N);
+
+	// 頂点 0 を根としたうえで,
+	// dp[i]: i を根とする部分木の頂点数
+	std::vector<int> dp(N);
+
+	DFS(graph, 0, -1, dp);
+
+	long long answer = 0;
+
+	// すべての辺について
+	for (int i = 0; i < (N - 1); ++i)
+	{
+		// 注目してる辺で葉側のグループの頂点数
+		const long long a = std::min(dp[A[i]], dp[B[i]]);
+
+		// その反対側のグループの頂点数
+		const long long b = (N - a);
+
+		// 組み合わせの数を加算
+		answer += (a * b);
+	}
+
+	std::cout << answer << '\n';
+}
+```
diff --git a/README.md b/README.md
index c9dfbd6..29f0b36 100644
--- a/README.md
+++ b/README.md
@@ -77,7 +77,7 @@ C++17 標準ライブラリの機能を優先して使い、競技プログラ
 |030| | | | |
 |[036](https://atcoder.jp/contests/typical90/tasks/typical90_aj)|[Max Manhattan Distance](./036.md)|★5|[👨‍🏫](https://raw.githubusercontent.com/E869120/kyopro_educational_90/main/editorial/036.jpg) / [📝](https://github.com/E869120/kyopro_educational_90/blob/main/sol/036.cpp)|マンハッタン距離は 45 度回転|
 |[037](https://atcoder.jp/contests/typical90/tasks/typical90_ak)|[Don't Leave the Spice](./037.md)|★5|[👨‍🏫](https://raw.githubusercontent.com/E869120/kyopro_educational_90/main/editorial/037.jpg) / [📝](https://github.com/E869120/kyopro_educational_90/blob/main/sol/037.cpp)|DP をセグメント木で高速化|
-|039| | | | |
+|[039](https://atcoder.jp/contests/typical90/tasks/typical90_am)|[Tree Distance](./039.md)|★5|[👨‍🏫](https://raw.githubusercontent.com/E869120/kyopro_educational_90/main/editorial/039.jpg) / [📝](https://github.com/E869120/kyopro_educational_90/blob/main/sol/039.cpp)|答えへの貢献度を考える|
 |051| | | | |
 |056| | | | |
 |[060](https://atcoder.jp/contests/typical90/tasks/typical90_bh)|[Chimera](./060.md)|★5|[👨‍🏫](https://raw.githubusercontent.com/E869120/kyopro_educational_90/main/editorial/060.jpg) / [📝](https://github.com/E869120/kyopro_educational_90/blob/main/sol/060.cpp)|両側から考える / 最長増加部分列|