哪位帮我看一下letters哪里错了，谢谢
http://ybt.ssoier.cn:8088/problem_show.php?pid=1212
#include<iostream>
#include<cstring>
using namespace std;
int s[4][2]={{-1,0},{1,0},{0,-1},{0,1}};
int n,m;
char a[25][25];
int mk[30];
int sum=0;
int MAX=-1;
void dfs(int x,int y){
for(int i=0;i<4;i++){
int xx=x+s[i][0];
int yy=y+s[i][1];
if(xx>=0&&xx<n&&yy>=0&&yy<m&&mk[a[xx][yy]-65]==0){
mk[a[xx][yy]-65]=1;
sum+=1;
dfs(xx,yy);
mk[a[xx][yy]-65]=0;
}
}
sum=0;
memset(mk,0,sizeof(mk));
if(sum>MAX){
MAX=sum;
}
return;
}
int main(){
cin>>n>>m;
for(int i=0;i<n;i++){
for(int j=0;j<m;j++){
cin>>a[i][j];
}
}
mk[a[0][0]-65]=0;
dfs(0,0);
cout<<MAX<<endl;
return 0;
}
