广搜做法:

#include <bits/stdc++.h>
using namespace std;
int n , m , b[25][25] , ans = 450 , next[4][2] = {{1 , 0} , 
                                                  {0 , 1} , 
                                                  {-1 , 0} , 
                                                  {0 , -1}};
char c[25][25];
struct point
{
    int x , y , time;
};
queue<point> s;
void bfs()
{
    while (s.size())
    {
        point now = s.front();
        s.pop();
        for (int i = 0 ; i < 4 ; i++)
        {
            int tx = now.x + next[i][0] , ty = now.y + next[i][1];
            if (tx >= 1 && tx <= n && ty >= 1 && ty <= m && b[tx][ty] == 0 && c[tx][ty] != '#')
            {
                if (c[tx][ty] == 'a')
                {
                    ans = min(ans , now.time + 1);
                }
                b[tx][ty] = 1;
                if (c[tx][ty] == 'x') s.push((point) {tx , ty , now.time + 2});
                else s.push((point) {tx , ty , now.time + 1});
            }
        }
    }
    return;
}

int main()
{
    cin >> n >> m;
    for (int i = 1 ; i <= n ; i++)
    {
        for (int j = 1 ; j <= m ; j++)
        {
            cin >> c[i][j];
            if (c[i][j] == 'r')
            {
                s.push((point) {i , j , 0});
                b[i][j] = 1;
            }
        }
    }
    bfs();
    if (ans == 450) cout << "Impossible";
    else cout << ans;
    return 0;
}//已AC