TLE in CSES Labyrinth

general
1

Problem link : CSES Labyrinth
My solution:

#include<bits/stdc++.h>
using namespace std;
#define pb push_back
#define f first
#define s second
#define mp make_pair
#define fast ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#define mod 1000000007
#define pi acos(-1)
typedef long long int ll;
typedef long double ld;
typedef vector<ld> vld;
typedef vector<ll> vll;
typedef pair<ll,ll> pll;
typedef vector<bool> vb;

bool isValid(ll x,ll y,vector<char> maze[],vector<vector<bool>> visited,ll n,ll m)
{
    if(x<0 || y<0 || x>=n || y>=m) return false;
    if(visited[x][y]==true) return false;
    if(maze[x][y]=='#') return false;
    return true;
}

void bfs(ll x1,ll y1,ll x2,ll y2,vector<char> maze[],ll n,ll m,
vector<vector<bool>> &visited,vector<vector<pll>> &parent,vector<pll> moves,ll &flag)
{
    queue<pll> q;
    q.push(mp(x1,y1));
    visited[x1][y1]=true;
    ll cx=x1,cy=y1;
    while(true)
    {
        for(auto x:moves)
        {
            ll a,b;
            a=x.f;
            b=x.s;
            if(isValid(cx+a,cy+b,maze,visited,n,m)==true)
            {
                parent[cx+a][cy+b]=mp(cx,cy);
                visited[cx+a][cy+b]=true;
                q.push(mp(cx+a,cy+b));
                if(maze[cx+a][cy+b]=='B')
                {
                    flag=1;
                    return;
                }
            }
        }
        if(q.empty()==1) return;
        else
        {
            pll next;
            next=q.front();
            q.pop();
            cx=next.f;
            cy=next.s;
        }
    }
}

void solve()
{
    ll n,m;
    cin>>n>>m;
    char a;
    vector<char> maze[n];
    vector<bool> v;
    ll x1,y1,x2,y2;
    for(ll i=0;i<n;i++)
    {
        for(ll j=0;j<m;j++)
        {
            cin>>a;
            maze[i].pb(a);
            if(a=='A')
            {
                x1=i;
                y1=j;
            }
            if(a=='B')
            {
                x2=i;
                y2=j;
            }
        }
    }

    vector<vector<bool>> visited(n,vector<bool> (m,false));
    vector<vector<pll>> parent(n,vector<pll> (m,mp(-1,-1)));
    vector<pll> moves;
    moves.pb(mp(-1,0));
    moves.pb(mp(1,0));
    moves.pb(mp(0,-1));
    moves.pb(mp(0,1));
    ll flag=0;
    bfs(x1,y1,x2,y2,maze,n,m,visited,parent,moves,flag);
    if(flag==0)
    {
        cout<<"NO\n";
        return;
    }
    else
    {
        cout<<"YES\n";
        string s="";
        ll x=x2,y=y2;
        while(true)
        {
            ll px,py;
            px=parent[x][y].f;
            py=parent[x][y].s;
            if(x-1==px)
            {
                s+='D';
            }
            else if(x+1==px)
            {
                s+='U';
            }
            else if(y-1==py)
            {
                s+='R';
            }
            else if(y+1==py)
            {
                s+='L';
            }
            x=px;
            y=py;
            if(maze[x][y]=='A') break;
        }
        cout<<s.length()<<"\n";
        for(ll i=s.length()-1;i>=0;i--) cout<<s[i];

    }
}

int main()
{
    fast;
    ll t=1;
    //cin>>t;
    while(t--)
    {
        solve();
    }
}

I am getting TLE in 5 test cases, the ones which have large matrices and a path does exist. What’s wrong in my solution?