My Solution :

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

void solve() {
int m , n;
cin >> m >> n;
vector<vector<char>> arr(m, vector<char>(n));
vector<vector<string>> vis(m, vector<string>(n, "X"));
int sx, sy , ex , ey;
for (int i = 0 ; i < m ; i++) {
for (int j = 0 ; j < n ; j++) {
char ch;
cin >> ch;
arr[i][j] = ch;
if (ch == 'A') {
sx = i;
sy = j;
}
if (ch == 'B') {
ex = i;
ey = j;
}
}
}
int dx[] = { -1, 1, 0, 0};
int dy[] = {0, 0, 1, -1};
queue<pair<int, int>> q;
q.push({sx, sy});
vis[sx][sy] = "";
while (!q.empty()) {
auto top = q.front();
q.pop();
int x = top.first;
int y = top.second;
// cout << x << " " << y << "\n";
for (int k = 0 ; k < 4 ; k++) {
int xx = x + dx[k];
int yy = y + dy[k];
if (xx >= 0 and xx<m and yy >= 0 and yy < n and vis[xx][yy] == "X" and arr[xx][yy] != '#') {
q.push({xx, yy});
if (k == 0) vis[xx][yy] = vis[x][y] + "U";
if (k == 1) vis[xx][yy] = vis[x][y] + "D";
if (k == 2) vis[xx][yy] = vis[x][y] + "R";
if (k == 3) vis[xx][yy] = vis[x][y] + "L";
}
}
}
if (vis[ex][ey] == "X") {
cout << "NO\n";
} else {
cout << "YES\n";
cout << int(vis[ex][ey].size()) << "\n";
cout << vis[ex][ey] << "\n";
}

}

int main() {
#ifndef ONLINE_JUDGE
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
solve();
return 0;
}

Dont know why i am getting Runtime error

Have you tried running the test cases on your own machine?

yes they are running

Have you checked that all of your vector indices are within bounds? You can replace the [] with .at() to make sure everything is within bounds.