input: Write a function to calculate the maximum possible revenue from items with given weights and values using the knapsack algorithm. The function should take two lists, weights and values, and an integer n representing the number of items. It should return the maximum revenue that can be achieved with a knapsack of capacity W=5.
  assert knapsack([2, 1, 3], [4, 2, 3], 3)==7
  assert knapsack([1, 2, 3], [6, 10, 12], 3)==22
  assert knapsack([1, 3, 4, 5], [1, 4, 5, 7], 4)==9
  assert knapsack([1, 1, 1], [10, 20, 30], 3)==50
  assert knapsack([], [], 0)==0
output: def knapsack(weights, values, n):
      W = 5
      dp = [0] * (W + 1)
      for i in range(n):
          for w in range(W, weights[i] - 1, -1):
              dp[w] = max(dp[w], dp[w - weights[i]] + values[i])
      return dp[W]