Abandoned warehouse

import sys
from collections import deque

def main():
    lines = sys.stdin.read().split()
    if not lines:
        return

    n = int(lines[0])
    m = int(lines[1])
    grid = lines[2:2 + n]

    start_r = start_c = -1
    end_r = end_c = -1

    for r in range(n):
        for c in range(m):
            if grid[r][c] == 'A':
                start_r, start_c = r, c
            elif grid[r][c] == 'B':
                end_r, end_c = r, c


    prev_move = [[None] * m for _ in range(n)]

    prev_move[start_r][start_c] = ''

    queue = deque([(start_r, start_c)])

    moves = [
        (-1, 0, 'U'),
        (1, 0, 'D'),
        (0, -1, 'L'),
        (0, 1, 'R')
    ]

    reached = False

    while queue:
        r, c = queue.popleft()

        if r == end_r and c == end_c:
            reached = True
            break

        for dr, dc, move in moves:
            nr, nc = r + dr, c + dc

            if 0 <= nr < n and 0 <= nc < m:
                if grid[nr][nc] != '#' and prev_move[nr][nc] is None:
                    prev_move[nr][nc] = move
                    queue.append((nr, nc))

    if not reached:
        print("NO")
        return

    path = []
    curr_r, curr_c = end_r, end_c

    rev_step = {
        'U': (1, 0),
        'D': (-1, 0),
        'L': (0, 1),
    }

    while (curr_r, curr_c) != (start_r, start_c):
        move = prev_move[curr_r][curr_c]
        path.append(move)
        dr, dc = rev_step[move]
        curr_r += dr
        curr_c += dc

    path.reverse()

    print("YES")
    print(len(path))
    print("".join(path))

main()

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 80, in <module>
    main()
  File "/box/input/code.py", line 70, in main
    dr, dc = rev_step[move]
KeyError: 'R'

Test 5

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 80, in <module>
    main()
  File "/box/input/code.py", line 70, in main
    dr, dc = rev_step[move]
KeyError: 'R'

Test 6

Verdict: ACCEPTED

Test 7

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 80, in <module>
    main()
  File "/box/input/code.py", line 70, in main
    dr, dc = rev_step[move]
KeyError: 'R'

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 80, in <module>
    main()
  File "/box/input/code.py", line 70, in main
    dr, dc = rev_step[move]
KeyError: 'R'

Test 12

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 80, in <module>
    main()
  File "/box/input/code.py", line 70, in main
    dr, dc = rev_step[move]
KeyError: 'R'

Test 13

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 80, in <module>
    main()
  File "/box/input/code.py", line 70, in main
    dr, dc = rev_step[move]
KeyError: 'R'

Test 14

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 80, in <module>
    main()
  File "/box/input/code.py", line 70, in main
    dr, dc = rev_step[move]
KeyError: 'R'

Test 15

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 80, in <module>
    main()
  File "/box/input/code.py", line 70, in main
    dr, dc = rev_step[move]
KeyError: 'R'

Test 16

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 80, in <module>
    main()
  File "/box/input/code.py", line 70, in main
    dr, dc = rev_step[move]
KeyError: 'R'