POJ 1154 LETTERS dfs

#include<iostream>
#include<cstdio>
#include<string>
#include<cstring>
#include<queue>
#define Max(a,b)a>b?a:b
using namespace std;
char map[22][22];
bool vs[22][22],v[30];
int R,C,ans;
int dir[4][2]={-1,0,1,0,0,1,0,-1};

void dfs(int x,int y,int step)
{
int k,p,np,nx,ny;
vs[x][y]=1;
p=map[x][y]-'A';
v[p]=1;
for(k=0;k<4;k++)
{
nx=x+dir[k][0];
ny=y+dir[k][1];
np=map[nx][ny]-'A';
if(nx<0||nx>=R||ny<0||ny>=C||vs[nx][ny]
||v[np])continue;
dfs(nx,ny,step+1);
}
ans=Max(ans,step);
vs[x][y]=0;
v[p]=0;
}
int main()
{
int i;
while(~scanf("%d%d",&R,&C))
{
for(i=0;i<R;i++)
{
getchar();
scanf("%s",map[i]);
}
memset(vs,0,sizeof(vs));
memset(v,0,sizeof(v));
ans=0;
dfs(0,0,1);
printf("%d\n",ans);
}
return 0;
}