input: Write a python function to find the longest common substring between two strings.
  assert longest_common_substring("abcdef", "zabcf") == "abc"
  assert longest_common_substring("12345", "51234") == "1234"
  assert longest_common_substring("helloworld", "worldhello") == "world"
output: def longest_common_substring(str1, str2):
      m, n = len(str1), len(str2)
      max_length = 0
      ending_index = 0
      dp = [[0] * (n + 1) for _ in range(m + 1)]
  
      for i in range(1, m + 1):
          for j in range(1, n + 1):
              if str1[i - 1] == str2[j - 1]:
                  dp[i][j] = dp[i - 1][j - 1] + 1
                  if dp[i][j] > max_length:
                      max_length = dp[i][j]
                      ending_index = i
  
      return str1[ending_index - max_length: ending_index]