poj 1154 LETTERS

#include<iostream>            //dfs
using namespace std;
bool visited[26];
char board[100][100];
int r,s,n;
bool vis(int x,int y)
{
    if(x<1||x>r||y<1||y>s)
        return true;
    return visited[board[x][y]-'A'];
}
void dfs(int x,int y,int num)
{
    if(vis(x-1,y)&&vis(x+1,y)&&vis(x,y-1)&&vis(x,y+1))
    {
        n=max(num,n);
        return ;
    }
    if(!vis(x-1,y))
    {
        visited[board[x-1][y]-'A']=1;
        dfs(x-1,y,num+1);
        visited[board[x-1][y]-'A']=0;
    }
    if(!vis(x+1,y))
    {
        visited[board[x+1][y]-'A']=1;
        dfs(x+1,y,num+1);
        visited[board[x+1][y]-'A']=0;
    }
    if(!vis(x,y-1))
    {
        visited[board[x][y-1]-'A']=1;
        dfs(x,y-1,num+1);
        visited[board[x][y-1]-'A']=0;
    }
    if(!vis(x,y+1))
    {
        visited[board[x][y+1]-'A']=1;
        dfs(x,y+1,num+1);
        visited[board[x][y+1]-'A']=0;
    }
}
int main()
{
    cin>>r>>s;
    for(int i=1;i<=r;++i)
        for(int j=1;j<=s;++j)
            cin>>board[i][j];
    visited[board[1][1]-'A']=1;
    dfs(1,1,1);
    cout<<n<<endl;
    return 0;
}