input: Write a function to find the longest common substring between two strings.
  assert longest_common_substring('people', 'talent') == 2
  assert longest_common_substring('abcdef', 'zabxy') == 2
  assert longest_common_substring('12345', '6789') == 0
output: def longest_common_substring(str1, str2):
      def lcs(X, Y, m, n, count):
          if m == 0 or n == 0:
              return count
          if X[m-1] == Y[n-1]:
              count = lcs(X, Y, m-1, n-1, count+1)
          count = max(count, max(lcs(X, Y, m, n-1, 0), lcs(X, Y, m-1, n, 0)))
          return count
  
      return lcs(str1, str2, len(str1), len(str2), 0)