input: Write a function to find the longest common substring in two given strings. The function should return the longest common substring. If the given strings have no common substring, return an empty string.
  assert longest_common_substring("Helwallomst", "Welcome")=="ello"
  assert longest_common_substring("abcde", "abc")=="abc"
  assert longest_common_substring("abc", "def")==""
  assert longest_common_substring("abcdef", "fedcba")=="a"
  assert longest_common_substring("same", "same")=="same"
output: def longest_common_substring(s1, s2):
      m, n = len(s1), len(s2)
      dp = [[0] * (n + 1) for _ in range(m + 1)]
      max_len = 0
      end_index = 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]