input: Write a function that takes two strings as arguments and returns the longest common substring between them. If there is no common substring, return an empty string.
  assert longest_common_substring("abcdef", "defg")=="def"
  assert longest_common_substring("abca", "bcaa")=="bca"
  assert longest_common_substring("xxx", "yyy")==""
  assert longest_common_substring("same", "same")=="same"
  assert longest_common_substring("", "any")==""
output: def longest_common_substring(s1, s2):
      m, n = len(s1), len(s2)
      dp = [[0] * (n + 1) for _ in range(m + 1)]
      max_len, end_index = 0, 0
      
      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]