input: Write a function to find the longest common substring in two given strings. If there is no common substring, return an empty string.
  assert longestCommonSubstring("abcdef", "bcdf")=="bcd"
  assert longestCommonSubstring("hello", "world")==""
  assert longestCommonSubstring("synchrony", "chrony")=="chrony"
  assert longestCommonSubstring("python", "typhon")=="ython"
  assert longestCommonSubstring("", "")==""
output: def longestCommonSubstring(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]