CSES - Monsters

#include <bits/stdc++.h>
using namespace std;

const int maxN = 1010;
char grid[maxN][maxN];
pair<int,int> parent[maxN][maxN];
vector<pair<int,int>> monsters;
int dx[8]= {1,0,-1,0,-1,-1,1,1};
int dy[8]= {0,1,0,-1,-1,1,-1,1};
char direction[4] = {'D','R','U','L'};
int cost_monsters[maxN][maxN],cost_player[maxN][maxN];
int end_x = -1,end_y = -1;

void BFS_Monsters (int n,int m) {
    for (int i = 1; i <= n; ++i) {
        for (int j = 1; j <= m; ++j) {
            cost_monsters[i][j] = -1;
        }
    }
    queue<pair<int,int>> q;
    for (int i = 0; i < (int) monsters.size(); ++i) {
        cost_monsters[monsters[i].first][monsters[i].second] = 0;
        q.push(monsters[i]);
    }
    while (!q.empty()) {
        int x = q.front().first, y = q.front().second;
        q.pop();
        for (int i = 0; i < 4; ++i) {
            int xu = x + dx[i],xv = y + dy[i];
            if (xu >= 1 and xu <= n and xv >= 1 and xv <= m and grid[xu][xv] != '#' and cost_monsters[xu][xv] == -1) {
                cost_monsters[xu][xv] = cost_monsters[x][y] + 1;
                q.push(make_pair(xu,xv));
            }
        }
    }
}

bool BFS_Player(int start_x,int start_y,int n,int m) {
    for (int i = 1; i <= n; ++i) {
        for (int j = 1; j <= m; ++j) {
            cost_player[i][j] = -1;
        }
    }
    cost_player[start_x][start_y] = 0;
    queue<pair<int,int>> q;
    q.push(make_pair(start_x,start_y));
    parent[start_x][start_y] = make_pair(-1,-1);
    while (!q.empty()) {
        int x = q.front().first, y = q.front().second;
        q.pop();
        for (int i = 0; i < 4; ++i) {
            int xu = x + dx[i], xv = y + dy[i];
            if (xu >= 1 and xu <= n and xv >= 1 and xv <= m and cost_player[xu][xv] == -1 and grid[xu][xv] == '.' and (cost_player[x][y] + 1 < cost_monsters[xu][xv] or cost_monsters[xu][xv] == -1)) {
                cost_player[xu][xv] = cost_player[x][y] + 1;
                q.push(make_pair(xu,xv));
                parent[xu][xv] = make_pair(x,y);
                if (xu == 1 or xu == n or xv == 1 or xv == m) {
                    end_x = xu,end_y = xv;
                    return true;
                }
            }
        }
    }
    return false;
}

int main () {
     ios::sync_with_stdio(false);
     cin.tie(nullptr);
     cout.tie(nullptr);
     int T = 1;
     //~ cin >> T;
     for (int test_case = 1; test_case <= T; ++test_case) {
         int n,m;
         cin >> n >> m;
         int start_x = -1, start_y = -1;
         for (int i = 1; i <= n; ++i) {
             for (int j = 1; j <= m; ++j) {
                 cin >> grid[i][j];
                 if (grid[i][j] == 'A') {
                     start_x = i,start_y = j;
                 }
                 if (grid[i][j] == 'M') {
                     monsters.push_back(make_pair(i,j));
                 }
             }
         }
         BFS_Monsters(n,m);
         if (!BFS_Player(start_x,start_y,n,m)) {
             if (start_x == 1 or start_x == n or start_y == 1 or start_y == m) {
                 cout << "YES\n" << 0 << '\n';
                 return 0;
             }
             cout << "NO\n";
             return 0;
         }
         pair<int,int> res = make_pair(end_x,end_y);
         string ans = "";
         vector<pair<int,int>> path;
         while (res != make_pair(-1,-1)) {
             path.push_back(res);
             res = parent[res.first][res.second];
         }
         reverse(path.begin(),path.end());
         for (int i = 0; i < (int) path.size(); ++i) {
             for (int j = 0; j < 4; ++j) {
                 if (path[i] == make_pair(start_x + dx[j],start_y + dy[j])) {
                     ans += direction[j];
                     start_x = path[i].first;
                     start_y = path[i].second;
                 }
             }
         }
         cout << "YES\n" << (int) ans.size() << '\n' << ans << '\n';
     }
     //cerr << "Time elapsed :" << clock() * 1000.0 / CLOCKS_PER_SEC << " ms" << '\n';
}