input: Write a function to find the longest common substring in two given strings. If no common substring exists, return an empty string.
  assert longestCommonSubstring("abcdef","abc")=="abc"
  assert longestCommonSubstring("abcdef","xyz")==""
  assert longestCommonSubstring("hello","world")=="o"
  assert longestCommonSubstring("","")==""
  assert longestCommonSubstring("exclusive","alternative")=="l"
output: def longestCommonSubstring(str1, str2):
      m, n = len(str1), len(str2)
      maxLength = 0
      endIndex = 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] > maxLength:
                      maxLength = dp[i][j]
                      endIndex = i
      
      return str1[endIndex - maxLength:endIndex]