Produces the same op ig, does not pass tests?

#!/bin/python3

import math
import os
import random
import re
import sys




first_multiple_input = input().rstrip().split()

n = int(first_multiple_input[0])

m = int(first_multiple_input[1])

matrix = []

for _ in range(n):
    matrix_item = input()
    matrix.append(matrix_item)

string = []
ans = ''
last_alnum = 0
chars = "!@#$%& "
for i in range(m):
    for j in range(n):
        string.append(matrix[j][i])

i = 0

while i < len( string ) :
    if string[i].isalnum():
        ans += string[i]
        i += 1
    else:
        j = i
        while j < len( string ) and not string[j].isalnum():
            j += 1
        if j == len(string):
            ans += ''.join(string[i:])
        else:
            ans += ' '
        i = j
print (ans)