input: Write a python function to find the longest common substring between two strings.
  assert findCommonSubstring("alexandra", "andromeda") == "andra"
  assert findCommonSubstring("happiness", "sappiness") == "appiness"
  assert findCommonSubstring("flight", "night") == "ight"
output: def findCommonSubstring(s1, s2):
      def longest_common_substr(s1, s2):
          m, n = len(s1), len(s2)
          dp = [[0] * (n + 1) for _ in range(m + 1)]
          length = 0
          end_pos = 0
  
          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] > length:
                          length = dp[i][j]
                          end_pos = i
                  else:
                      dp[i][j] = 0
  
          return s1[end_pos - length:end_pos]
  
      return longest_common_substr(s1, s2)