input: Write a python function to find the longest common sub-string in two strings.
  assert LongestCommonString("OldSite:Geeksforgeeks.org", "Newsite:Geeks.com") == "Geeks"
  assert LongestCommonString("abcdxyz", "xyzabcd") == "abcd"
  assert LongestCommonString("zxabcdezy", "yzabcdezx") == "abcdez"
output: def LongestCommonString(str1, str2):
      m = len(str1)
      n = len(str2)
      result = 0
      end = 0
      length = [[0] * (n + 1) for _ in range(m + 1)]
  
      for i in range(m + 1):
          for j in range(n + 1):
              if i == 0 or j == 0:
                  length[i][j] = 0
              elif str1[i - 1] == str2[j - 1]:
                  length[i][j] = length[i - 1][j - 1] + 1
                  if length[i][j] > result:
                      result = length[i][j]
                      end = i - 1
              else:
                  length[i][j] = 0
      
      if result == 0:
          return ""
      
      return str1[end - result + 1: end + 1]