input: Write a function to find the longest common subsequence (LCS) of two strings.
  assert find_lcs("ACBA", "ABAC") == "ABA"
  assert find_lcs("ABCBDAB", "BDCAB") == "BCAB"
  assert find_lcs("XMJYAUZ", "MZJAWXU") == "MJAU"
  assert find_lcs("AGGTAB", "GXTXAYB") == "GTAB"
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:
                  if len(dp[i - 1][j]) > len(dp[i][j - 1]):
                      dp[i][j] = dp[i - 1][j]
                  else:
                      dp[i][j] = dp[i][j - 1]
  
      return dp[m][n]