Project Euler Problem 18 Solution

Project Euler & HackerRank Problem 18 Solution

Maximum path sum I
by {BetaProjects} | MAY 17, 2009 | Project Euler & HackerRank

Project Euler Problem 18 Statement

By starting at the top of the triangle below and moving to adjacent numbers on the row below, the maximum total from top to bottom is 23.

             3
            7 5
           2 4 6
          8 5 9 3
That is, 3 + 7 + 4 + 9 = 23.

Find the maximum total from top to bottom of the triangle below:

        75
      95 64
     17 47 82
    18 35 87 10
  20 04 82 47 65
… data continues

NOTE: As there are only 16384 routes, it is possible to solve this problem by trying every route. However, Problem 67, is the same challenge with a triangle containing one–hundred rows; it cannot be solved by brute force, and requires a clever method!

Solution

To solve this problem and problem 67, which is much larger, start the search from the bottom to the top, adding the maximums along the way. This will “bubble” the maximum path total to the top of the triangle.

Let’s follow this technique, step–by–step, with the 4 row triangle example above to show how this works.

A step–by–step look at this algorithm

Initial array           After the first 
                           iteration
      3                        3
     7 5                      7 5
    2 4 6                   10 13 15
   8 5 9 3
  1. Starting at the bottom, take the first pair, 8 and 5, pick the maximum and replace the 2 in the previous row with their sum 8+2=10.
  2. Take the next pair, 5 and 9, pick the maximum and replace the 4 in the previous row with their sum 9+4=13.
  3. Finally, take the last pair, 9 and 3, pick the maximum and replace the 6 in the previous row with their sum 9+6=15.

If you think about it, we simply solved the maximum path from the last row to the second to the last row by considering each sub–problem for the following three triangles.

   2      4      6
  8 5    5 9    9 3

Keep that in mind and let’s do it again with our new array.

  1. Take the larger of 10 and 13 and add it to 7 making 13+7=20.
  2. Take the larger of 13 and 15 and add it to 5 making 15+5=20.

Now our array looks like:

  3
20 20

At last we take the larger of 20 and 20 (yes, I know they’re the same) and add it to 3 making 20+2=23.
And our array looks like:

23

Which is the maximum total path in the triangle.

And if you follow this logic then you just witnessed dynamic programming in action—truly short and simple.

HackerRank version

HackerRank Project Euler 18 varies the number of rows in the triangle from 1 ≤ N ≤ 15 and runs 10 test cases. No changes required except to read from std input instead of a file.

Python Source Code

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