BZOJ 1644 Usaco2007 Oct Obstacle Course 障碍训练课 SPFA

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#define M 10100
#define P(x,y) ((x)*n-n+(y))
using namespace std;
const int dx[]={0,0,1,-1};
const int dy[]={1,-1,0,0};
int n,A,B;
int map[M],f[M<<2];
void SPFA()
{
	static int q[65540];
	static bool v[M<<2];
	static unsigned short r,h;
	int i;
	memset(f,0x3f,sizeof f);
	for(i=0;i<4;i++)
		f[q[++r]=A<<2|i]=0,v[A<<2|i]=true;
	while(r!=h)
	{
		int x=q[++h];v[x]=false;
		int _x=((x>>2)-1)/n+1,_y=((x>>2)-1)%n+1,_dir=x&3;
		for(i=0;i<4;i++)
		{
			int xx=_x+dx[i],yy=_y+dy[i];
			if(xx<=0||yy<=0||xx>n||yy>n)
				continue;
			if(map[P(xx,yy)]) continue;
			int y=P(xx,yy)<<2|i;
			if(f[y]>f[x]+(i!=_dir) )
			{
				f[y]=f[x]+(i!=_dir);
				if(!v[y])
					v[y]=true,q[++r]=y;
			}
		}
	}
}
int main()
{
	int i,j;
	cin>>n;
	for(i=1;i<=n;i++)
	{
		static char s[M];
		scanf("%s",s+1);
		for(j=1;j<=n;j++)
			switch(s[j])
			{
				case 'x':
					map[P(i,j)]=1;
					break;
				case 'A':
					A=P(i,j);
					break;
				case 'B':
					B=P(i,j);
					break;
			}
	}
	SPFA();
	cout<<min(min(f[B<<2|0],f[B<<2|1]),min(f[B<<2|2],f[B<<2|3]))<<endl;
	return 0;
}