백준 1987번 알파벳

https://www.acmicpc.net/problem/1987

#include <iostream>
#include <cmath>
using namespace std;
char board[20][20] = {0, };
bool step[26] = {0, };
int row, col, answer = 1;
int dx[4] = {0, -1, 0, 1};
int dy[4] = {-1, 0, 1, 0};
bool isInner(int x, int y) {
    return (x >= 0 && x < row && y >= 0 && y < col);
}
void move(int depth, int x, int y) {
    answer = answer > depth ? answer : depth;
    
    for (int i = 0; i < 4; ++i) {
        int newX = x + dx[i];
        int newY = y + dy[i];
        
        if (isInner(newX, newY) && 
            !step[board[newX][newY] - 'A']) {\
            
            step[board[newX][newY] - 'A'] = true;
            move(depth + 1, newX, newY);
            step[board[newX][newY] - 'A'] = false;
        }
    }
}
int main()
{
    int result = 0;
    
    scanf("%d %d", &row, &col);
    
    for (int i = 0 ; i < row; ++i) {
        scanf("%s", &board[i]);
    }
    
    step[board[0][0] - 'A'] = true;
    move(1, 0, 0);
    printf("%d\n", answer);
 
    return 0;
}