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2 points by lacker 5982 days ago | link | parent

No prob. Sorry if I go on at too much length here ;-)

So a useful formula for counting digits is, a positive integer x has floor(log10(x)) + 1 digits. You can figure this out yourself by thinking, where does the digit-counting function bump up a notch? At 10, 100, 1000, etc. So asymptotically the number of digits in x is O(log x).

So n! has O(log n!) digits. The trickier part is figuring out or knowing that O(n log n) = O(log n!). Using log ab = log a + log b you can expand out the factorial:

  O(log n!) = O(log (n * (n-1) * ... * n * 1))
            = O(log n + log (n-1) + ... + log 2 + log 1)
            = O(n log n)
In case the last step isn't clear, you can do this splitting-in-half bounding trick. Since each element in the sum is less than log n you can bound from above with

  log n + log (n-1) + ... + log 2 + log 1 < n log n
And if you just take the larger half of the list you can bound from below with

  log n + log (n-1) + ... + log 2 + log 2 > log n + log (n-1) + ... + log (n/2)
                                          > (n/2) log (n/2)
which is itself O(n log n). So O(log n!) = O(n log n).

In general the rules of thumb you use to reduce O(log n!) are:

  1. complicated expressions inside factorials are ugly, you should simplify them
  2. O(sum of n things) is usually O(n * the biggest thing)
Make sense?