Shared code

#include<bits/stdc++.h>
using namespace std;
class Monsters{
  int n,m,Ai,Aj;
  vector<vector<char>>labyrinth;
  vector<vector<int>>prev;
  vector<vector<bool>>vis;
  vector<int>X,Y;
  queue<array<int,3>>que; 
   bool isValid(int x,int y){
      return x>=0 and x<n and y>=0 and y<m and labyrinth[x][y]!='#' and !vis[x][y];
    }
  public:
    Monsters(int n,int m,vector<vector<char>>&labyrinth){
       this->n=n;
       this->m=m;
       this->labyrinth=labyrinth;
       vis.resize(n,vector<bool>(m,0));
       prev.resize(n,vector<int>(m,-1));
       X={1,0,-1,0};
       Y={0,1,0,-1};
       for(int i=0;i<n;i++){
        for(int j=0;j<m;j++){
          if(labyrinth[i][j]=='M'){
            que.push({i,j,0});
            vis[i][j]=true;
          }else if(labyrinth[i][j]=='A'){
            Ai=i;
            Aj=j;
            vis[i][j]=true;
          }
        }
       }
    }  
    bool isGoal(int i,int j){
      return (i==0 or i==n-1 or j==0 or j==m-1) and (labyrinth[i][j]=='.' or labyrinth[i][j]=='A');
    }
   string canReach(){
      int x,y;
      int goalX=-1,goalY=-1,len;
      que.push({Ai,Aj,1});
      while(!que.empty()){
        len=que.size();
        while(len--){
        auto pos=que.front();
        que.pop();
         if(pos[2]==1 and isGoal(pos[0],pos[1])){
            goalY=pos[1];
            goalX=pos[0];
             break;  
            }
        for(int dir=0;dir<4;dir++){
          x=pos[0]+X[dir];
          y=pos[1]+Y[dir];
          if(isValid(x,y)){
            vis[x][y]=true;
            que.push({x,y,pos[2]});
            if(pos[2]==1)prev[x][y]=dir;
            }
           }
         }
        }
      if(goalY==-1){
        return "NO";
      }   
      string direction;
      while(goalX!=Ai or goalY!=Aj){
         if(prev[goalX][goalY]==0)direction+="D";
         else if(prev[goalX][goalY]==1)direction+="R";
         else if(prev[goalX][goalY]==2)direction+="U";
         else direction+="L";
         x=goalX;
         y=goalY;
          goalX-=X[prev[x][y]];
          goalY-=Y[prev[x][y]];
      }
      reverse(direction.begin(),direction.end());
      return direction;
   }
};
int main(){
  int n,m;
  cin>>n>>m;
  vector<vector<char>>labyrinth(n,vector<char>(m));
  for(int i=0;i<n;i++){
    for(int j=0;j<m;j++){
      cin>>labyrinth[i][j];
    }
  }
Monsters sol(n,m,labyrinth);
string ans=sol.canReach();
if(ans=="NO"){
  cout<<ans<<endl;
}
else{
  cout<<"YES\n"<<ans.size()<<endl<<ans<<endl;
}
return 0;
}