Luogu2986 [USACO10MAR]伟大的奶牛聚集 （树形DP）

有点权的重心，拆掉点dfs不就是了吗

//#include <iostream>

#include <cstdio>

#include <cstring>

//#include <algorithm>

//#include <cmath>

#define R(a,b,c) for(register int  a = (b); a <= (c); ++ a)

#define nR(a,b,c) for(register int  a = (b); a >= (c); -- a)

#define Max(a,b) ((a) > (b) ? (a) : (b))

#define Min(a,b) ((a) < (b) ? (a) : (b))

#define Fill(a,b) memset(a, b, sizeof(a))

#define Abs(a) ((a) < 0 ? -(a) : (a))

#define Swap(a,b) a^=b^=a^=b

#define ll long long

#define ON_DEBUG

#ifdef ON_DEBUG

#define D_e_Line printf("\n\n----------\n\n")

#define D_e(x)  cout << #x << " = " << x << endl

#define Pause() system("pause")

#define FileOpen() freopen("in.txt","r",stdin);

#else

#define D_e_Line ;

#define D_e(x)  ;

#define Pause() ;

#define FileOpen() ;

#endif

struct ios{

    template<typename ATP>ios& operator >> (ATP &x){

        x = 0; int f = 1; char c;

        for(c = getchar(); c < '0' || c > '9'; c = getchar()) if(c == '-')  f = -1;

        while(c >= '0' && c <= '9') x = x * 10 + (c ^ '0'), c = getchar();

        x*= f;

        return *this;

    }

}io;

using namespace std;

const int N = 100007;

struct Edge{

	int nxt, pre, w;

}e[N << 1];

int head[N], cntEdge;

inline void add(int u, int v, int w){

	e[++cntEdge] = (Edge){ head[u], v, w}, head[u] = cntEdge;

}

int C[N], totSize;

int siz[N];

long long f[N];

inline void DFS_1(int u, int fa){

	siz[u] = C[u];

	for(register int i = head[u]; i; i = e[i].nxt){

		int v = e[i].pre;

		if(v == fa) continue;

		DFS_1(v, u);

		siz[u] += siz[v];

		f[u] += f[v] + 1ll * siz[v] * e[i].w;

	}

}

long long ans = 9223372036854775807;

inline void DFS_2(int u, int fa){

	for(register int i = head[u]; i; i = e[i].nxt){

		int v = e[i].pre;

		if(v == fa) continue;

		f[v] = f[u] - 1ll * siz[v] * e[i].w + 1ll * (totSize - siz[v]) * e[i].w;

		ans = Min(ans, f[v]);

		DFS_2(v, u);

	}

}

int main(){

//FileOpen();

	int n;

	io >> n;

	R(i,1,n){

		io >> C[i];

		totSize += C[i];

	}

	R(i,2,n){

		int u, v, w;

		io >> u >> v >> w;

		add(u, v, w);

		add(v, u, w);

	}

//	DFS_First(1, 0);

//	DFS_Second(1, 1);

//

//	long long ans = 9223372036854775807;

//	R(i,1,n){

//		long long sum = 0;

//		R(j,1,n){

//			if(i == j) continue;

//			sum += 1ll * (dis[i] - (dis[LCA(i, j)] << 1) + dis[j]) * C[j];

//		}

//		ans = Min(ans, sum);

//	}

	DFS_1(1, 0);

	for(register int i = 2; i <= n; i += 3) f[i] = f[i + 1] = f[i + 2] = 0;

	DFS_2(1, 0);

	printf("%lld", ans);

	return 0;

}

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