poj 1383 Labyrinth 树的直径

//poj 1383
//sep9
#include <iostream>
using namespace std;
const int maxN=1024;
char g[maxN][maxN];
int d[maxN][maxN];
int r,c;
int dx[6]={0,0,1,-1};
int dy[6]={1,-1,0,0};
void dfs(int px,int py,int x,int y,int sum)
{
	d[x][y]=sum;
	for(int t=0;t<4;++t){
		int newx=x+dx[t];
		int newy=y+dy[t];
		if(newx>=0&&newx<r&&newy>=0&&newy<c&&g[newx][newy]=='.'&&(newx!=px||newy!=py))
			dfs(x,y,newx,newy,sum+1);
	}	
}
int main()
{
	int cases;
	scanf("%d",&cases);
	while(cases--){
		int i,j;
		memset(d,0,sizeof(d));
                scanf("%d%d",&c,&r);
		for(i=0;i<r;++i)
			scanf("%s",g[i]);
		int ans=0;
		int sx=-1,sy=-1;
		for(i=0;i<r&&sx==-1;++i)
			for(j=0;j<c&&sx==-1;++j)
				if(g[i][j]=='.'){
					sx=i;
					sy=j;
				}
		dfs(-1,-1,sx,sy,0);	
		int maxx=-1,maxi,maxj;
		for(i=0;i<r;++i)
			for(j=0;j<c;++j)
				if(maxx<d[i][j]){
					maxx=d[i][j];
					maxi=i;
					maxj=j;
				}
		memset(d,0,sizeof(d));	
		dfs(-1,-1,maxi,maxj,0);
		for(i=0;i<r;++i)
			for(j=0;j<c;++j)
				ans=max(ans,d[i][j]);
		printf("Maximum rope length is %d.\n",ans);	
	}
	return 0;	
}