Project Euler – Lattice Path (Part 6)

This will be my last blog post on the lattice path problem for now. As always, full credit goes to Pim Spelier.

After all that theory I want to do some reading on real life applications of lattice paths.

Also I have some other blog topics and Project Euler problems on my mind.

The final solution to the lattice path problem is shockingly simply, if you know your math. I fiddled with a similar idea when I started working on this, but failed to figure it out in the end. Because I don’t know my math (yet).

Enter combinatorics….

Let’s have a look at the 2 x 2 grid. Whatever path you take from the top left to the bottom right corner, you always have to go the same number of steps right- and downwards.


So it is either RRDD, RDRD, RDDR, DRRD, DRDR or DDRR for the 2 x 2 grid.

Always a combination of two steps to the right and two steps to the bottom. The number of steps total is defined by the grid size. 2 x 2 = 4 in this example.

For a grid of width n and height m it’s n-times R and m-times D with a total step length of n x m.

Now all you have to figure out is, how many unique combinations of Rs and Ds in a string of length n x exist.

Actually you only have to know the unique ways of placing Rs in a string of length n x m. Once the Rs are in place, the Ds are automatically in place as well.

The math you need to figure this one out is called binomial coefficient, short nCr for from n choose r.

How many unique ways can two Rs placed in a string of length 4?
It is from 4 choose 2, which is 6:


Several ways to compute the binomial coefficient exist, one of them being a multiplicative formula:


Our grid is a perfect square (2 x 2, 3 x 3, 4 x 4…20 x 20). As shown above for a 2 x 2 grid it’s from 4 choose 2. For a 3 x 3 grid it’s from 6 choose 3 and so on. Therefore the relation between n and k is 2n to n:


And a sum can be implemented using a for-loop:


Done! Now I need to install me some Latex. Powerpoint is such a pain.

Project Euler – Lattice Path (Part 5)

Apart from the recursive solutions an iterative approach for solving the lattice path problem exists as well.

Again full credit goes to Pim Spelier. I really need to find this guy online. So I can properly link to him.

All my work is to translate the given algorithm to Java and dissect it in a way, so that I can understand the solution.

Without further ado…


The algorithm performs nicely within a blink of a second. Even if I increase the grid size above 20.

So how does it work?

First we create a two-dimensional array. Let’s use the 2×2 grid as an example.
As of long[][] grid = new long[gridHeight + 1][gridWidth + 1] our empty two-dimensional array looks like this:


Actually all grid cells hold an initial value of 0.

Next step is to fill the grid partially:

for (int i = 0; i <= gridHeight; i++) {
    grid[i][0] = 1;

for (int j = 0; j <= gridWidth; j++) {
    grid[0][j] = 1;

two_dimensional_array_partially_filledAnd then we fill in the remaining slots:

for (int i = 1; i <= gridHeight; i++) {
    for (int j = 1; j <= gridWidth; j++) {
        grid[i][j] = grid[i – 1][j] + grid[i][j – 1];


After the first iteration (i = 1 and j = 1) the grid looks like this:


After the second iteration (i = 1 and j = 2):


Third iteration (i = 2 and j = 1):


And finally (i = 2 and j = 2), which is exactly Pascal’s triangle:


There is one more blog post to write about this problem. It will be about a combinatorial solution.

Project Euler – Lattice Path (Part 4)

My blog post Project Euler – Lattice Path (Part 3) showed an implementation of a recursive solution to the lattice path problem in Java.

Major problem was the performance of the algorithm. Working fine for small grids, but lasting forever for the actual 20 x 20 grid.

The recursive nature of the algorithm creates a tree structure.

Unfortunately, already computed values are computed again.


This is no big deal for a 2 x 2 grid. Only the value for (1, 1), which is 2, gets computed twice.

But for a larger grid, walking all tree branches without a sanity check (“Hey, this road looks familiar. I must have been here before.”), the repeated recursive computation causes the performance issue.

For a 3 x 3 grid the values for (2, 1), (1, 1), (2, 2) and (1, 2) have to be computed twice.

For a 4 x 4 grid it’s the values for (3, 1), (2, 1), (1, 1), (3, 2), (2, 2), (1, 2), (3, 3), (2, 3) and (1, 3).

For a 5 x 5 grid…you get the idea.

Pim Spelier introduces an optimization for the recursive algorithm, memoization:

In computing, memoization is an optimization technique used primarily to speed up computer programs by storing the results of expensive function calls and returning the cached result when the same inputs occur again. (Wikipedia)

So let’s try caching.


Et voilà, down to 0 seconds from previously 10+ minutes.

Project Euler – Pythagorean Triplet

While working on another blog post about the lattice path problem my hard drive suffered from a terminal meltdown. Sadly all work lost.

Tomorrow I will take my iMac to my local Apple Store’s Genius Bar.

Meanwhile, while watching Zoolander on TV (what a great movie by the way), I took a quick shot at another Project Euler problem about Pythagorean triplets.


I did some research about constructing Pythagorean triplets, but in the end I had to realize that a very simple brute force solution exists.


There might be better way to do this. But I keep this for another blog post. Typing on my laptop is way less fun and Hansel requires my attention anyway.

Project Euler – Lattice Path (Part 3)

I use a 2×2 grid trying to explain a recursive solution to problem number 15.

Credit for the algorithm goes to Pim Spelier, who wrote a nice solution on Project Euler, which is downloadable as PDF once you have submitted the correct answer.

The idea is to assign a tuple to each grid point, starting with (0,0) at the upper left corner. First row (0,0), (0,1), (0,2). Second row (1,0), (1,1)…you get the idea. The bottom right corner is (2,2).

Now we turn the grid into Pascal’s triangle.


Looking at the grid this way, it is now more transparent how the grid actually is a Pascal’s triangle. Starting from the top left corner, only one possible route exists to each grid point marked with a “1”. A grid point marked with a “2” can be reached by taking two different routes. Guess how many routes lead to a grid point with a “3”.

The number of lattice paths through the grid is the sum of the paths to (2,1) and (1,2): 3 + 3 = 6.

You can see that for every tuple containing a 0 like (0,0) or (2.0) and so on, we must return 1.

Every other grid point number is the sum of the two numbers directly above it.

This leads to the following recursive algorithm.


The function is called 11 times.


Unfortunately this solution has a “minor” flaw, which is performance. Calculating the lattice path for a 20×20 grid on my iMac G5 takes over 10 minutes.


A better performing algorithm will be the topic of an upcoming blog post.

Project Euler – Lattice Path (Part 2)

Based on the formula for calculating the central binomial coefficient I quickly hacked together some code.


While writing this post I already know that much nicer recursive and iterative algorithms exists. I will turn them into another blog post. But first I have to understand those solutions.

I was not sure about wether to put this post out or not. In the end I decided to publish it because:

  • Arriving at solution is a process. I want to keep this notion of progress.
  • Nothing is perfect right from the start. Analyzing forever will not get anything done.
  • A neat and nearly perfect blog post in the end does not reflect the effort taken to understand this problem. It’s like social media where a picture posted on Facebook looks awesome to make everybody in your friendlist jealous, but is actually far from reality.
  • I want to keep this as real as possible. If I make a fool of myself, so be it. At least I am a curious fool ;-).