blog/algorithms/04-recursion/2023-08-17-pyramid-slide-down/03-top-down-dp.md

id	slug	title	description	tags	last_update
top-down-dp	/recursion/pyramid-slide-down/top-down-dp	Top-down DP solution	Top-down DP solution of the Pyramid Slide Down.	java dynamic-programming top-down-dp	date 2023-08-17

Top-down dynamic programming

Top-down dynamic programming is probably the most common approach, since (at least looks like) is the easiest to implement. The whole point is avoiding the unnecessary computations that we have already done.

In our case, we can use our naïve solution and put a cache on top of it that will make sure, we don't do unnecessary calculations.

// This “structure” is required, since I have decided to use ‹TreeMap› which
// requires the ordering on the keys. It represents one position in the pyramid.
record Position(int row, int col) implements Comparable<Position> {
    public int compareTo(Position r) {
        if (row != r.row) {
            return Integer.valueOf(row).compareTo(r.row);
        }

        if (col != r.col) {
            return Integer.valueOf(col).compareTo(r.col);
        }

        return 0;
    }
}

public static int longestSlideDown(
        int[][] pyramid,
        TreeMap<Position, Integer> cache,
        Position position) {
    int row = position.row;
    int col = position.col;

    if (row >= pyramid.length || col < 0 || col >= pyramid[row].length) {
        // BASE: out of bounds
        return Integer.MIN_VALUE;
    }

    if (row == pyramid.length - 1) {
        // BASE: bottom of the pyramid
        return pyramid[position.row][position.col];
    }

    if (!cache.containsKey(position)) {
        // We haven't computed the position yet, so we run the same “formula” as
        // in the naïve version »and« we put calculated slide into the cache.
        // Next time we want the slide down from given position, it will be just
        // retrieved from the cache.
        int slideDown = Math.max(
                longestSlideDown(pyramid, cache, new Position(row + 1, col)),
                longestSlideDown(pyramid, cache, new Position(row + 1, col + 1)));
        cache.put(position, pyramid[row][col] + slideDown);
    }

    return cache.get(position);
}

public static int longestSlideDown(int[][] pyramid) {
    // At the beginning we need to create a cache and share it across the calls.
    TreeMap<Position, Integer> cache = new TreeMap<>();
    return longestSlideDown(pyramid, cache, new Position(0, 0));
}

You have probably noticed that record Position have appeared. Since we are caching the already computed values, we need a “reasonable” key. In this case we share the cache only for one run (i.e. pyramid) of the longestSlideDown, so we can cache just with the indices within the pyramid, i.e. the Position.

:::tip Record

Record is relatively new addition to the Java language. It is basically an immutable structure with implicitly defined .equals(), .hashCode(), .toString() and getters for the attributes.

:::

Because of the choice of TreeMap, we had to additionally define the ordering on it.

In the longestSlideDown you can notice that the computation which used to be at the end of the naïve version above, is now wrapped in an if statement that checks for the presence of the position in the cache and computes the slide down just when it's needed.

Time complexity

If you think that evaluating time complexity for this approach is a bit more tricky, you are right. Keeping the cache in mind, it is not the easiest thing to do. However there are some observations that might help us figure this out:

1. Slide down from each position is calculated only once.
2. Once calculated, we use the result from the cache.

Knowing this, we still cannot, at least easily, describe the time complexity of finding the best slide down from a specific position, but we can bound it from above for the whole run from the top. Now the question is how we can do that!

Overall we are doing the same things for almost¹ all of the positions within the pyramid:

1. We calculate and store it (using the partial results stored in cache). This is done only once.

   For each calculation we take 2 values from the cache and insert one value. Because we have chosen TreeMap, these 3 operations have logarithmic time complexity and therefore this step is equivalent to 3 \cdot \log_2{n}.

   However for the sake of simplicity, we are going to account only for the insertion, the reason is rather simple, if we include the 2 retrievals here, it will be interleaved with the next step, therefore it is easier to keep the retrievals in the following point.

   :::warning[caution]

   You might have noticed it's still not that easy, cause we're not having full cache right from the beginning, but the sum of those logarithms cannot be expressed in a nice way, so taking the upper bound, i.e. expecting the cache to be full at all times, is the best option for nice and readable complexity of the whole approach.

   :::

   Our final upper bound of this work is therefore \log_2{n}.

2. We retrieve it from the cache. Same as in first point, but only twice, so we get 2 \cdot \log_2{n}.

   :::warning[caution]

   It's done twice because of the .containsKey() in the if condition.

   :::

Okay, we have evaluated work done for each of the cells in the pyramid and now we need to put it together.

Let's split the time complexity of our solution into two operands:

\mathcal{O}(r + s)

r will represent the actual calculation of the cells and s will represent the additional retrievals on top of the calculation.

We calculate the values only once, therefore we can safely agree on:

\begin{align*} r &= n \cdot \log{n} \ \end{align*}

What about the s though? Key observation here is the fact that we have 2 lookups on the tree in each of them and we do it twice, cause each cell has at most 2 parents:

\begin{align*} s &= n \cdot 2 \cdot \left( 2 \cdot \log{n} \right) \ s &= 4 \cdot n \cdot \log{n} \end{align*}

:::tip

You might've noticed that lookups actually take more time than the construction of the results. This is not entirely true, since we have included the .containsKey() and .get() from the return statement in the second part.

If we were to represent this more precisely, we could've gone with:

\begin{align*} r &= 3 \cdot n \cdot \log{n} \ s &= 2 \cdot n \cdot \log{n} \end{align*}

On the other hand we are summing both numbers together, therefore in the end it doesn't really matter.

(Feel free to compare the sums of both “splits”.)

:::

And so our final time complexity for the whole top-down dynamic programming approach is:

\mathcal{O}(r + s) \ \mathcal{O}(n \cdot \log{n} + 4 \cdot n \cdot \log{n}) \ \mathcal{O}(5 \cdot n \cdot \log{n}) \ \mathcal{O}(n \cdot \log{n})

As you can see, this is worse than our greedy solution that was incorrect, but it's better than the naïve one.

Memory complexity

With this approach we need to talk about the memory complexity too, because we have introduced cache. If you think that the memory complexity is linear to the input, you are right. We start at the top and try to find each and every slide down. At the end we get the final result for new Position(0, 0), so we need to compute everything below.

That's how we obtain:

\mathcal{O}(n)

n represents the total amount of cells in the pyramid, i.e.

\sum_{y=0}^{\mathtt{pyramid.length} - 1} \mathtt{pyramid}\left[y\right]\mathtt{.length}

:::warning[caution]

If you're wondering whether it's correct because of the second if in our function, your guess is right. However we are expressing the complexity in the Bachmann-Landau notation, so we care about the upper bound, not the exact number.

:::

:::tip Can this be optimized?

Yes, it can! Try to think about a way, how can you minimize the memory complexity of this approach. I'll give you a hint:

\mathcal{O}(rows)

:::

---

1. except the bottom row ↩︎