学步园

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

先求一次最大流，再交换b1,b2求一次最大流

注意交换的技巧

//#define _TEST _TEST
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <iostream>
#include <cmath>
#include <algorithm>
using namespace std;
/************************************************
Code By willinglive    Blog:http://willinglive.cf
************************************************/
#define rep(i,l,r) for(int i=(l),___t=(r);i<=___t;i++)
#define per(i,r,l) for(int i=(r),___t=(l);i>=___t;i--)
#define MS(arr,x) memset(arr,x,sizeof(arr))
#define LL long long
#define INE(i,u,e) for(int i=head[u];~i;i=e[i].next)
inline const int read()
{int r=0,k=1;char c=getchar();for(;c<'0'||c>'9';c=getchar())if(c=='-')k=-1;
for(;c>='0'&&c<='9';c=getchar())r=r*10+c-'0';return k*r;}
/////////////////////////////////////////////////
const int inf=0x3f3f3f3f;
int n,a1,a2,an,b1,b2,bn;

struct edge{int v,flow,cap,next;}e[8000];
int head[60],k,_k;
int S,T;
int d[60],q[60],cur[60],tmp[60];
/////////////////////////////////////////////////
inline void adde(int u,int v,int f){e[k].v=v;e[k].flow=0;e[k].cap=f;e[k].next=head[u];head[u]=k++;}
void ins(int u,int v,int f1,int f2){adde(u,v,f1);adde(v,u,f2);}
inline int F(int i){return e[i].cap-e[i].flow;}
bool bfs()
{
    MS(d,-1); d[q[0]=S]=0; memcpy(cur,head,sizeof(cur));
    for(int l=0,r=1;l<r;)
    {
    	int u=q[l++],v;
    	INE(i,u,e) if(F(i) && d[v=e[i].v]==-1)
    	{
    		q[r++]=v; d[v]=d[u]+1;
    		if(v==T) return 1;
    	}
    }
    return 0;
}
int dfs(int u,int a)
{
	if(u==T||a==0) return a;
	for(int &i=cur[u];~i;i=e[i].next) if(d[e[i].v]==d[u]+1 && F(i))
	{
		if(int t=dfs(e[i].v,min(a,F(i))))
		{
			e[i].flow+=t; e[i^1].flow-=t;
			return t;
		}
	}
	return 0;
}
int dinic()
{
	int f=0;
	while(bfs()) while(int t=dfs(S,inf)) f+=t;
	return f;
}
/////////////////////////////////////////////////
void input()
{
	MS(head,-1); k=_k=0;
    a1++;a2++;b1++;b2++;an<<=1;bn<<=1;
    char s[55];
    rep(i,1,n)
    {
    	scanf("%s",s+1);
    	rep(j,1,i-1)
    	{
    		if(s[j]=='N')
    		{
    			ins(i,j,inf,inf);
    		}
    		else if(s[j]=='O')
    		{
    			ins(i,j,2,2);
    		}
    	}
    }
    S=n+1,T=n+2;
}
void solve()
{
    _k=k; memcpy(tmp,head,sizeof(tmp));
    
    ins(S,a1,an,0); ins(S,b1,bn,0);
    ins(a2,T,an,0); ins(b2,T,bn,0);
    if(dinic()!=an+bn) {puts("No");return;}
    
    k=_k; memcpy(head,tmp,sizeof(tmp));
    
    ins(S,a1,an,0); ins(S,b2,bn,0);
    ins(a2,T,an,0); ins(b1,T,bn,0);
    rep(i,0,k) e[i].flow=0;
    if(dinic()!=an+bn) {puts("No");return;}
    puts("Yes");
}
/////////////////////////////////////////////////
int main()
{
    #ifndef _TEST
    freopen("bridge.in","r",stdin); freopen("bridge.out","w",stdout);
    #endif
    while(~scanf("%d%d%d%d%d%d%d",&n,&a1,&a2,&an,&b1,&b2,&bn))
    input(),solve();
    return 0;
}