神仙网络流QAQ

这个主要还是没想通

环是啥：图中全是入读出度为1的点

所以用费用流表示改变方向

#include<bits/stdc++.h>
using namespace std;
const int N=1e5+10;
const int INF=0x3f3f3f3f;
struct Front_star{
	int u,v,w,c,nxt;
}e[N<<2];
int cnt=1;
int first[N]={};
void addedge(int u,int v,int w,int c){
	cnt++;
	e[cnt].u=u;
	e[cnt].v=v;
	e[cnt].w=w;
	e[cnt].c=c;
	e[cnt].nxt=first[u];
	first[u]=cnt;
}
void add(int u,int v,int w,int c){
	addedge(u,v,w,c);
	addedge(v,u,0,-c);
}
int R,C;
int idx[21][21]={};
int idxcnt=0;
int pos[21][21]={};
int S=0,T=1001;
int dx[5]={0,0,0,-1,1};
int dy[5]={0,-1,1,0,0};
map<int,int> mmp;
void Build_Gra(){
	for(int i=1;i<=R;i++){
		for(int j=1;j<=C;j++){
			idx[i][j]=++idxcnt;
		}
	}
	for(int i=1;i<=R;i++){
		for(int j=1;j<=C;j++){
			for(int k=1;k<=4;k++){
				int xx=i+dx[k];
				int yy=j+dy[k];
				xx=(xx-1+R)%R+1;
				yy=(yy-1+C)%C+1;
				add(idx[i][j],idx[xx][yy]+R*C,1,(pos[i][j]==k?0:1));
			}
		}
	}	
	for(int i=1;i<=R;i++){
		for(int j=1;j<=C;j++){
			add(S,idx[i][j],1,0);
			add(idx[i][j]+R*C,T,1,0);
		}
	}
}
queue<int> Q;
int inqueue[N]={};
int dis[N]={};
int pre[N]={};
bool SPFA(){
	for(int i=S;i<=T;++i)dis[i]=INF,pre[i]=0;
	dis[S]=0;
	Q.push(S);
	while(!Q.empty()){
		int x=Q.front();
		Q.pop();
		inqueue[x]=0;
		for(int i=first[x];i;i=e[i].nxt){
			int v=e[i].v;
			if(e[i].w&&e[i].c+dis[x]<dis[v]){
				pre[v]=i;
				dis[v]=dis[x]+e[i].c;
				if(!inqueue[v]){
					inqueue[v]=1;
					Q.push(v);
				}
			}
		}
	}
	return dis[T]!=INF;
}
void MCMF(){
	int ans=0;
	while(SPFA()){
		int s=INF;
		for(int i=pre[T];i;i=pre[e[i^1].v])s=min(s,e[i].w);
		for(int i=pre[T];i;i=pre[e[i^1].v]){
			e[i].w-=s;
			e[i^1].w+=s;
		}
		ans+=s*dis[T];
	}
	cout<<ans;
}
int main(){
//	freopen("test.in","r",stdin);
//	freopen("test.out","w",stdout); 
	scanf("%d%d",&R,&C);
	T=R*C*2+1;
	mmp['L']=1;
	mmp['R']=2;
	mmp['U']=3;
	mmp['D']=4;
	for(int i=1;i<=R;i++){
		char S[21];
		scanf("%s",S+1);
		for(int j=1;j<=C;j++){
			pos[i][j]=mmp[S[j]];		
		}
	}
	Build_Gra();
	MCMF();
	return 0;
}