bzoj3504: [Cqoi2014]危桥

传送门：http://http://www.lydsy.com/JudgeOnline/problem.php?id=3504

思路：证明见此

http://blog.csdn.net/wzq_QwQ/article/details/46546977

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
const int maxn=110,maxm=10010,inf=1e9;
using namespace std;
int n,a1,a2,b1,b2,an,bn,map[maxn][maxn],S=maxn-2,T=maxn-1;char ch[maxn];bool cant;
int pre[maxm],now[maxn],son[maxm],val[maxm],tot=1,head,tail,q[maxn+10],dis[maxn];
void add(int a,int b,int c){pre[++tot]=now[a],now[a]=tot,son[tot]=b,val[tot]=c;}
void ins(int a,int b,int c){add(a,b,c),add(b,a,0);}
void build(){
	memset(now,0,sizeof(now)),tot=1;
	for (int i=1;i<=n;i++)
		for (int j=1;j<=n;j++)
			if (map[i][j]==1) ins(i,j,2);
			else if (map[i][j]==2) ins(i,j,inf);
}

bool bfs(){
	memset(dis,-1,sizeof(dis));
	q[tail=1]=S,dis[S]=head=0;
	while (head!=tail){
		if (++head>maxn) head=1;
		int x=q[head];
		for (int y=now[x];y;y=pre[y]) if (dis[son[y]]==-1&&val[y]){
			if (++tail>maxn) tail=1;
			dis[son[y]]=dis[x]+1,q[tail]=son[y];
		}
	}
	return dis[T]>0;
}

int find(int x,int low){
	if (x==T) return low;
	int y,res=0;
	for (y=now[x];y;y=pre[y]){
		if (dis[son[y]]!=dis[x]+1||!val[y]) continue;
		int tmp=find(son[y],min(low,val[y]));
		res+=tmp,low-=tmp,val[y]-=tmp,val[y^1]+=tmp;
		if (!low) break;
	}
	if (!y) dis[x]=-1;
	return res;
}

int main(){
	while (scanf("%d%d%d%d%d%d%d",&n,&a1,&a2,&an,&b1,&b2,&bn)!=EOF){
		a1++,b1++,a2++,b2++,cant=0;
		memset(map,0,sizeof(map));
		for (int i=1;i<=n;i++){
			scanf("%s",ch+1);
			for (int j=1;j<=n;j++){
				if (ch[j]=='O') map[i][j]=1;
				else if (ch[j]=='N') map[i][j]=2;
			}
		}
		build(),ins(S,a1,an*2),ins(a2,T,an*2),ins(S,b1,bn*2),ins(b2,T,bn*2);
		int ans=0;
		while (bfs()) ans+=find(S,inf);
		if (ans<(an+bn)*2) cant=1;
		if (!cant){
			build();
			ins(S,a1,an*2),ins(a2,T,an*2),ins(S,b2,bn*2),ins(b1,T,bn*2);
			ans=0;while (bfs()) ans+=find(S,inf);
			if (ans<(an+bn)*2) cant=1;
		}
		puts(cant?"No":"Yes");
	}
	return 0;
}