树形DP 洛谷P1131

猛戳这里

我咋认为这题是贪心呢…

//MADE BY Y_is_sunshine;
//#include <bits/stdc++.h>
//#include <memory.h>
#include <algorithm>
#include <iostream>
#include <cstdlib>
#include <cstring>
#include <sstream>
#include <cstdio>
#include <vector>
#include <string>
#include <cmath>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <math.h>

#define INF 0x3f3f3f3f
#define MAXN 5000005

const int mod = 1e9 + 7;

using namespace std;

struct node {
	int to;
	int next;
	int val;
}edge[MAXN << 1];

int N, M;
int cnt;
int sz[MAXN];
int from[MAXN];
int price[MAXN];

void add(int x, int y, int z) {
	edge[++cnt].next = from[x];
	edge[cnt].to = y;
	edge[cnt].val = z;
	from[x] = cnt;
}

void dfs(int u, int fa) {
	for (int i = from[u]; i; i = edge[i].next) {
		int v = edge[i].to;
		if (v == fa)
			continue;
		dfs(v, u);
		price[u] = max(price[u], price[v] + edge[i].val);
	}
}

long long sum = 0;

void solve(int u, int fa) {
	for (int i = from[u]; i; i = edge[i].next) {
		int v = edge[i].to;
		if (v == fa)
			continue;
		solve(v, u);
		sum += price[u] - price[v] - edge[i].val;
	}
}

int main()
{
	freopen("data.txt", "r", stdin);

	cin >> N;
	int T;
	cin >> T;
	for (int i = 1; i < N; i++) {
		int a, b, c;
		scanf("%d%d%d", &a, &b, &c);
		add(a, b, c);
		add(b, a, c);
	}

	dfs(T, -1);
	solve(T, -1);

	cout << sum << endl;


	freopen("CON", "r", stdin);
	system("pause");
	return 0;
}