最后两个测试例 都比较坑，需要特判，附带样例

import pprint
r,c=map(int,input().split())
b=[]
for x in range(c):
    b.append(list(input()))
_=input()
d=[(1,0),(0,1),(-1,0),(0,-1)]

node=[]
vis=set()

for x in range(c):
    for y in range(r):
        if b[x][y]=='@':
            node.append((x,y))
            vis.add((x,y))
            break
    else:
        continue
    break
                 
step=1
def bfs():
    global step
    while node:
        size=len(node)
        for i in range(size):
            x=node[0][0]
            y=node[0][1]
            node.remove((x,y))
            for m,n in d:
               
                if 0<=x+m<c and 0<=y+n<len(b[x+m]) and (x+m,y+n) not in vis and b[x+m][y+n]!="#":
                    if y+n>=c:
                        continue
                    step+=1
                    node.append((x+m,y+n))
                    vis.add((x+m,y+n))
     
bfs()
print(step)

最后一个样例：

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