(C++) - 백준(BOJ) 1987번 : 알파벳 답

#include <iostream>
using namespace std;
char chess[21][21];
int d[21][21];
int ck[21][21];
int dx[4] = { 0,0,-1,1 };
int dy[4] = { -1,1,0,0 };
int R, C;
bool f[26];
int ans;
void dfs(int i,int j) {
    ans = max(ans, d[i][j]);
    for (int t = 0; t < 4; t++)
    {
        int nx = i + dx[t];
        int ny = j + dy[t];
        if (1 <= nx && nx <= R && 1 <= ny && ny <= C)
        {
            if (ck[nx][ny] == 0 && f[chess[nx][ny]-'A']==0)//갈수있는데 체크가 안되어있다면
            {
                ck[nx][ny] = 1;
                f[chess[nx][ny] - 'A'] = 1;//그 전 알파벳을 저장
                d[nx][ny] = d[i][j] + 1;
                dfs(nx, ny);
                ck[nx][ny] = 0;
                f[chess[nx][ny] - 'A'] = 0;
            }
        }
    }
 
}
int main() {
    cin >> R >> C;
    for (int i = 1; i <= R; i++)
    {
        for (int j = 1; j <= C; j++)
        {
            cin >> chess[i][j];
        }
    }
    ck[1][1] = 1;
    d[1][1] = 1;
    f[chess[1][1] - 'A'] = 1;
    dfs(1,1);
 
    cout << ans << '\n';
}