input: Write a python function to find the length of the longest common subsequence between two strings.
  assert longest_subsequence("abcde", "ace")==3
  assert longest_subsequence("abc", "abc")==3
  assert longest_subsequence("abc", "def")==0
output: def longest_subsequence(x, y):
      m, n = len(x), len(y)
      dp = [[0] * (n + 1) for _ in range(m + 1)]
      
      for i in range(1, m + 1):
          for j in range(1, n + 1):
              if x[i - 1] == y[j - 1]:
                  dp[i][j] = dp[i - 1][j - 1] + 1
              else:
                  dp[i][j] = max(dp[i - 1][j], dp[i][j - 1])
      
      return dp[m][n]