ac代码
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<iostream>
using namespace std;
char Map[30][30];
int vis[27];
int m,n,cnt;
int dr[]={-1,0,1,0};
int dc[]={0,1,0,-1};
void dfs(int r,int c,int depth)
{
        vis[Map[r][c]-'A']=1;
        int flag=0;
        for(int i=0;i<=3;i++)
        {
                int new_r=r+dr[i];
                int new_c=c+dc[i];
                if(new_r>n||new_r<1||new_c<1||new_c>m)  continue;
                if(!vis[Map[new_r][new_c]-'A'])
                {
                  flag=1;
                  dfs(new_r,new_c,depth+1);
                }
        }
        vis[Map[r][c]-'A']=0;
        if(!flag){cnt=max(cnt,depth);}
}
int main()
{
        while(scanf("%d%d",&n,&m)==2)
        {
                cnt=0;
                memset(vis,0,sizeof(vis));
                getchar();
                for(int i=1;i<=n;i++)
                  {
                        for(int j=1;j<=m;j++)
                        scanf("%c",&Map[i][j]);
                        getchar();
                }
        dfs(1,1,1);
        printf("%d\n",cnt);
        }
}