input: Write a function to find the longest common substring in two strings. If both strings are empty then return an empty string.
  assert longest_common_substring("Helwallomst", "Welcome")=="l"
  assert longest_common_substring("abcd", "abc")=="abc"
  assert longest_common_substring("abcdef", "defghi")=="def"
  assert longest_common_substring("abc", "xyz")==""
  assert longest_common_substring("", "")==""
output: def longest_common_substring(s1, s2):
      if not s1 or not s2:
          return ""
      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]