input: Write a function to find the longest common substring in two strings and return the substring. If no common substring is found, return an empty string.
  assert longestCommonSubstring("abcdef", "bcdf")=="bcd"
  assert longestCommonSubstring("xyz", "abc")==""
  assert longestCommonSubstring("hello", "world")=="o"
  assert longestCommonSubstring("abcdef", "xyz")==""
  assert longestCommonSubstring("", "")==""
output: def longestCommonSubstring(str1, str2):
      m, n = len(str1), len(str2)
      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 str1[i - 1] == str2[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 str1[end_index - max_len:end_index]