909. Snakes and Ladders

class Solution {
    pair<int,int> coordinate(int x,int n){
      int r=n-(x-1)/n-1;
      int c=(x-1)%n;
      if(r%2==n%2){
          return {r,n-c-1};
      }
      return {r,c};
    }
public:
    int snakesAndLadders(vector<vector<int>>& board) {
        int n=board.size();
        int m=board[0].size();
        int moves=0;
        queue<int> q;
        vector<vector<bool>> vis(n,vector(n,false));
        vis[n-1][0]=true;
        q.push(1);
        while(!q.empty()){
            int size=q.size();
            for(int i=0;i<size;i++){
            int x=q.front();
            if(x==n*n){
                return moves;
            }
            q.pop();
            for(int j=1;j<=6;j++){
                if(j+x>n*n){
                    break;
                }
                pair<int,int> coor=coordinate(x+j,n);
                if(vis[coor.first][coor.second]){
                    continue;
                }
                vis[coor.first][coor.second]=true;
                if(board[coor.first][coor.second]!=-1){
                    q.push(board[coor.first][coor.second]);
                }else{
                    q.push(j+x);
                }
            }
        }
          moves++;
        }
        return -1;
    }
};