input: Write a function to find the longest common substring in two given strings. The function should return the longest common substring. If there is no common substring, it should return an empty string.
  assert longest_common_substring("Helwallomst", "Welcome")=="l"
  assert longest_common_substring("abcde", "cdeab")=="cde"
  assert longest_common_substring("abcdef", "ghijkl")==""
  assert longest_common_substring("", "test")==""
  assert longest_common_substring("hello", "world")=="l"
output: def longest_common_substring(s1, s2):
      m, n = len(s1), len(s2)
      max_len = 0
      end_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 s1[i - 1] == s2[j - 1]:
                  dp[i][j] = dp[i - 1][j - 1] + 1
                  if dp[i][j] > max_len:
                      max_len = dp[i][j]
                      end_index = i
      
      return s1[end_index - max_len:end_index]