LuoguP1131 [ZJOI2007]时态同步（树形DP，贪心)

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#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 = 500007;

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;

}

long long ans;

int dis[N];

inline void DFS(int u, int fa){

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

		int v = e[i].pre;

		if(v == fa) continue;

		DFS(v, u);

		dis[u] = Max(dis[u], dis[v] + e[i].w);

	}

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

		int v = e[i].pre;

		if(v == fa) continue;

		ans += dis[u] - dis[v] - e[i].w;

	}

}

int main(){

	int n, rt;

	io >> n >> rt;

	R(i,2,n){

		int u, v, w;

		io >> u >> v >> w;

		add(u, v, w);

		add(v, u, w);

	}

	DFS(rt, 0);

	printf("%lld", ans);

	return 0;

}

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