Suppose that we say: 

Cons lst = null;
for (int i = 0; i < n; i++ )
    lst = cons( i, lst );
lst = nreverse(lst);

What is the Big O of this code?

  • A: O(1)
  • B: O(n)
  • C: O(n log n)
  • D: O(n2)
  • E: error

    Answer: B

    The cons is always O(1), inside a loop of n, so the loop is O(n). nreverse is also O(n). O(n)   +   O(n)   =   O(n).

    Rule: If the order in which you cons up a list makes it come out backwards, nreverse it at the end.

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