input: Write a python function to find the longest common substring between two given strings.
  assert longest_common_substring("abcdef", "zabcyydef") == "abc"
  assert longest_common_substring("helloworld", "worldhello") == "world"
  assert longest_common_substring("abcdxyz", "xyzabcd") == "abcd"
  assert longest_common_substring("abcdefgh", "ijklmnop") == ""
output: def longest_common_substring(s1, s2):
      m = len(s1)
      n = len(s2)
      result = ""
      longest_length = 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 s1[i - 1] == s2[j - 1]:
                  dp[i][j] = dp[i - 1][j - 1] + 1
                  if dp[i][j] > longest_length:
                      longest_length = dp[i][j]
                      result = s1[i - 1 - longest_length + 1:i]
              else:
                  dp[i][j] = 0
  
      return result