poj1154 【DFS】

//#include<bits/stdc++.h>
#include<iostream>
#include<cstdio>
#include<string>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<map>
#include<queue>
#include<stack>
#include<set>
#include<list>
using namespace std;
#define ll long long
const int Mod = 1e9+7;
const int inf = 1e9;
const int Max = 1e5+10;
vector<int>vt[Max];
int dx[] = {-1, 1,  0, 0};
int dy[] = { 0, 0, -1, 1};
//void exgcd(ll a,ll b,ll& d,ll& x,ll& y){if(!b){d=a;x=1;y=0;}else{exgcd(b,a%b,d,y,x);y-=x*(a/b);}}
//ll inv(ll a,ll n){ll d, x, y;exgcd(a,n,d,x,y);return (x+n)%n;}  ???
//int gcd(int a,int b)  {  return (b>0)?gcd(b,a%b):a;  }  ??С???
//int lcm(int a, int b)  {  return a*b/gcd(a, b);   }    ??С????
int ans = 1,n,m,vis[25];
char mp[25][25];

void dfs(int x,int y,int cnt){
    for(int i=0;i<4;i++){
        int nx = dx[i] + x;
        int ny = dy[i] + y;
        if(nx>=0&&nx<n&&ny>=0&&ny<m&&vis[mp[nx][ny]-‘A‘]==0){
            vis[mp[nx][ny]-‘A‘] = 1;
            ans = max(ans,cnt+1);
            dfs(nx,ny,cnt+1);
            vis[mp[nx][ny]-‘A‘] = 0;
        }
    }
}

int main()
{
    cin>>n>>m;
    memset(vis,0,sizeof(vis));
    for(int i=0;i<n;i++){
        for(int j=0;j<m;j++){
            cin>>mp[i][j];
        }
    }
    vis[mp[0][0]-‘A‘] = 1;
    dfs(0,0,1);
    cout<<ans<<endl;
}