input: Write a python function to find the longest common subsequence between two strings.
  assert longestCommonSubsequence("ABCD", "BCEA") == "BC"
  assert longestCommonSubsequence("AGGTAB", "GXTXAYB") == "GTAB"
  assert longestCommonSubsequence("ABC", "AC") == "AC"
  assert longestCommonSubsequence("ABC", "DEF") == ""
output: def longestCommonSubsequence(str1, str2):
      m, n = len(str1), len(str2)
      dp = [["" for _ in range(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] + str1[i - 1]
              else:
                  dp[i][j] = dp[i - 1][j] if len(dp[i - 1][j]) > len(dp[i][j - 1]) else dp[i][j - 1]
  
      return dp[m][n]