Question

I’m teaching myself dynamic programming. It’s almost magical. But seriously. Anyway, the problem I worked out was : Given a stairs of N steps and a child who can either take 1, 2, or 3 steps at a time, how many different ways can the child reach the top step?. The problem wasn’t too hard, my implementation is below.

import java.util.HashMap;

public class ChildSteps {
    private HashMap<Integer, Integer> waysToStep;

    public ChildSteps() {
        waysToStep = new HashMap<Integer, Integer>();
    }

    public int getNthStep(int n) {
        if (n < 0) return 0; // 0 ways to get to a negative step

        // Base Case
        if (n == 0) return 1;

        // If not yet memorized
        if (!waysToStep.containsKey(n)) {
            waysToStep.put(n, getNthStep(n - 3) + getNthStep(n - 2) + getNthStep(n - 1));
        }

        return waysToStep.get(n);
    }
}

However, now I want to get the runtime. How should I figure this out? I am familiar (and not much more) with Akra-Bazzi and Master Theorem. Do those apply here?

http://en.wikipedia.org/wiki/Master_theorem

Here it would seem that it could be: T(N) = 3 * T(???) + O(1) but I’m really not sure.

thanks guys.

Answer 1

In a worst case scenario analysis it would be:

T(N) = N * (containsKey(N) + 8)

Assuming that containsKey = N (it is probably N^2 or Log(N)) then this simplifies to T(N) = N.

You would have to find out the function for containsKey(N) to get the actual equation.

You’re really over thinking this though; you don’t need to do a algorithm analysis for this. Good quote for you: “Premature optimization is the root of all evil”