bfs로 풀어봤는데 뭐가 문제인지 모르겠어요

import sys
from collections import deque
sys.stdin = open("input.txt", 'r')
dx = [-1, 0, 1, 0]
dy = [0, 1, 0, -1]
n = int(input())
board = [list(map(int, input().split())) for _ in range(n)]
res = 0
dq = deque()
for h in range(100):
    ch = [[0]*n for _ in range(n)]
    cnt = 0
    for i in range(n):
        for j in range(n):
            if board[i][j] > h:
                dq.append((i, j))
                ch[i][j] = 1
                while dq:
                    tmp = dq.popleft()
                    for k in range(4):
                        x = tmp[0]+dx[k]
                        y = tmp[1]+dy[k]
                        if 0 <= x < n and 0 <= y < n and ch[x][y] == 0 and board[x][y] > h:
                            ch[x][y] = 1
                            dq.append((x, y))
                cnt += 1
    res = max(res, cnt)
    if cnt == 0:
        break
print(res)