BZOJ2435【TreeDP】

/* I will wait for you*/

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <ctime>
#include <algorithm>
#include <iostream>
#include <fstream>
#include <vector>
#include <queue>
#include <deque>
#include <map>
#include <set>
#include <string>
#define make make_pair
#define fi first
#define se second

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;

const int maxn = 1000010;
const int maxm = 1010;
const int maxs = 26;
const int inf = 0x3f3f3f3f;
const int P = 1000000007;
const double error = 1e-9;

inline int read()
{
	int x = 0, f = 1;
	char ch = getchar();
	while (ch <= 47 || ch >= 58)
		f = (ch == 45 ? -1 : 1), ch = getchar();
	while (ch >= 48 && ch <= 57)
		x = x * 10 + ch - 48, ch = getchar();
	return x * f;
}

struct edge
{
	int v, w, next;
} e[2 * maxn];

int n, cnt, head[maxn], size[maxn]; ll ans;

void insert(int u, int v, int w)
{
	e[cnt] = (edge) {v, w, head[u]}, head[u] = cnt++;
	e[cnt] = (edge) {u, w, head[v]}, head[v] = cnt++;
}

void dfs(int u, int p, int w)
{
	for (int i = head[u], v; i != -1; i = e[i].next)
		if ((v = e[i].v) != p) dfs(v, u, e[i].w), size[u] += size[v];
	ans += (ll) w * abs(n - 2 *(++size[u]));
}

int main()
{
	n = read(), memset(head, -1, sizeof head);
	for (int i = 1, u, v, w; i < n; i++)
		u = read(), v = read(), w = read(), insert(u, v, w);
	dfs(1, 0, 0), printf("%lld\n", ans);

	return 0;
}

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