input: Write a function to calculate the maximum possible revenue from the items with given weights and values using the knapsack problem approach. The function should take in a list of weights, a list of values, and the number of items, and return the maximum revenue for a knapsack with a specific capacity.
  
  assert knapsack([2, 1, 3], [4, 2, 3], 3)==7
  assert knapsack([1, 2, 3], [6, 10, 12], 3)==22
  assert knapsack([1, 2], [4, 2], 2)==6
  assert knapsack([1, 1, 1], [1, 2, 3], 3)==6
  assert knapsack([], [], 0)==0
output: def knapsack(weights, values, n):
      capacity = 5  # Assuming a fixed capacity for simplicity
      dp = [0] * (capacity + 1)
      
      for i in range(n):
          for w in range(capacity, weights[i] - 1, -1):
              dp[w] = max(dp[w], dp[w - weights[i]] + values[i])
      
      return dp[capacity]