网易笔试第四题的思路是什么呀？

def distance(arr,i,j):
    if i<0 or i>=len(arr) or j<0 or j>=len(arr[0]):
        return 0
    if arr[i][j] == 0:
        return 0
    else:
        return 1+min(distance(arr,i-1,j),distance(arr,i+1,j),distance(arr,i,j-1),distance(arr,i,j+1))

def init_2d(m,n):
    d = []
    for i in range(m):
        d.append([])
        for j in range(n):
            d[i].append(0)
    return d

def distance(arr,i,j):
    if i<0 or i>=len(arr) or j<0 or j>=len(arr[0]):
        return 0
    if arr[i][j] == 0:
        return 0
    else:
        return 1+min(distance(arr,i-1,j),distance(arr,i+1,j),distance(arr,i,j-1),distance(arr,i,j+1))

line1 = input().split(" ")
m,n = int(line1[0]),int(line1[1])

d = init_2d(m,n)

for i in range(m):
    line = input()
    for j,s in enumerate(line):
        d[i][j] = int(s)
# print(distance(d,0,2))
# print(d)
arr1 = init_2d(m,n)
for i in range(m):
    for j in range(n):
        arr1[i][j] = distance(d,i,j)
print(arr1)