【NOI2008】志愿者招募

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【NOI2008】志愿者招募

费用流+建图

详细题解

备用题解

就看看代码吧。

#include<bits/stdc++.h>
using namespace std;
#define ll long long
const int inf=(1<<30);
const int N=1e3+10;
const int M=1e4+5; 
struct edge{
	int v,f,c,nxt;
}e[M<<2];
int first[N],cnt=1;
inline void add(int u,int v,int f,int c){
	e[++cnt].v=v;e[cnt].f=f;e[cnt].c=c;
	e[cnt].nxt=first[u];first[u]=cnt;
	e[++cnt].v=u;e[cnt].f=0;e[cnt].c=-c;
	e[cnt].nxt=first[v];first[v]=cnt;//
}
int n,m,a[N],vis[N],dis[N],s,t;
bool spfa(){
	memset(vis,0,sizeof(vis));
	memset(dis,0x3f,sizeof(dis));
	deque<int>q;
	q.push_back(t);
	dis[t]=0;vis[t]=1;
	while(!q.empty()){
		int u=q.front();q.pop_front();vis[u]=0;
		for(int i=first[u];i;i=e[i].nxt){
			int v=e[i].v;
			if(e[i^1].f>0&&dis[v]>dis[u]-e[i].c){//
				dis[v]=dis[u]-e[i].c;
				if(!vis[v]){
					vis[v]=1;
					if(!q.empty()&&dis[v]<dis[q.front()])q.push_front(v);
					else q.push_back(v);
				}
			}
		}
	}
	return dis[s]<0x3f3f3f3f;
}
int money=0;
int dfs(int x,int f){
	if(x==t){vis[t]=1;return f;}
	int used=0,k;vis[x]=1;
	for(int i=first[x];i;i=e[i].nxt){
		int v=e[i].v;
		if(e[i].f<1||vis[v]||dis[x]!=dis[v]+e[i].c)continue;
		k=dfs(v,min(e[i].f,f-used));//
		if(!k)continue;
		money+=k*e[i].c;
		e[i].f-=k;e[i^1].f+=k;
		used+=k;
		if(used==f)break;
	}
	return used;
}
int ZKW(){
	int flow=0;
	while(spfa()){
		vis[t]=1;
		while(vis[t]){
			memset(vis,0,sizeof(vis));
			flow+=dfs(s,inf);
		}
	}
	return flow;
}
int main(){
	scanf("%d%d",&n,&m);
	for(int i=1;i<=n;i++)
		scanf("%d",&a[i]);
	for(int i=2;i<=n+1;i++)add(i,i-1,inf,0);//
	for(int i=1;i<=m;i++){
		int u,v,w;
		scanf("%d%d%d",&u,&v,&w);
		add(u,v+1,inf,w);
	}
	s=0;t=n+2;
	for(int i=1;i<=n+1;i++){
		int c=a[i]-a[i-1];
		if(c>0)add(s,i,c,0);
		if(c<0)add(i,t,-c,0);
	}
	ZKW();
	printf("%d",money);
	return 0;
}