input: Write a function to find the longest common subsequence (LCS) of two given strings. The function should return the LCS as a string. If the input strings are empty, the function should return an empty string.
  
  assert find_lcs("abcdaf", "acbcf")=="abcf"
  assert find_lcs("aa", "xaay")=="aa"
  assert find_lcs("abc", "def")==""
  assert find_lcs("", "xyz")==""
  assert find_lcs("pqrst", "")==""
output: def find_lcs(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] = max(dp[i - 1][j], dp[i][j - 1], key=len)
      
      return dp[m][n]