Abandoned warehouse

from collections import deque
import sys

sys.setrecursionlimit(700)


def bfs(x, y, output):
    moves[x][y] = "X"
    possible_moves = [(1, 0, "D"), (0, -1, "L"), (-1, 0, "U"), (0, 1, "R")]
    end = False
    q = deque([(x, y)])
    while q:
        x, y = q.popleft()
        if graph[x][y] == "B":
            end = True
            break
        visited[x][y] = True
        for dx, dy, move in possible_moves:
            new_x, new_y = x + dx, y + dy
            if (
                0 <= new_x < n
                and 0 <= new_y < m
                and graph[new_x][new_y] != "#"
                and not visited[new_x][new_y]
            ):
                visited[new_x][new_y] = True
                moves[new_x][new_y] = move
                q.append((new_x, new_y))

    if end:
        back_path(x, y, output)
        return True
    else:
        return False


def task3(n, m, graph):
    output = []
    for i in range(n):
        for j in range(m):
            if graph[i][j] == "A":
                start = (i, j)
            elif graph[i][j] == "B":
                dest = (i, j)

    if bfs(start[0], start[1], output):
        print("YES")
        print(len(output))
        output = reversed(output)
        print("".join(output))
    else:
        print("NO")


def back_path(x, y, output):
    if moves[x][y] != "X":
        move = moves[x][y]
        output.append(move)
        if move == "U":
            back_path(x + 1, y, output)
        elif move == "D":
            back_path(x - 1, y, output)
        elif move == "L":
            back_path(x, y + 1, output)
        elif move == "R":
            back_path(x, y - 1, output)


if __name__ == "__main__":
    n, m = map(int, input().split())
    graph = [input().strip() for i in range(n)]
    visited = [[False] * m for i in range(n)]
    moves = []
    moves = [[""] * m for i in range(n)]
    task3(n, m, graph)

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 75, in <module>
    task3(...

Test 12

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 75, in <module>
    task3(...

Test 13

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 75, in <module>
    task3(...

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 75, in <module>
    task3(...