[Community Puzzle] The River I

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Hi all,

So I’ve been playing on Codingame lately and have really fallen in love with the platform while I’ve been using it to get myself up to speed on C#. Unfortunately I’m backed into a corner now.

I’m coding up The River I in C#, using queues to track the sequences, and it seems a fairly straightforward puzzle. It took me about 20 minutes to get a basic solution coded locally that passed all the tests, along with all the validators… except for the last one, “Even Bigger”. That last one just silently fails and stubbornly laughs at me.

I can’t for the life of me figure out where the code could be failing, and as I said, it tackles every other test without issue. I’m even clearing the queues as I go, to lighten it up on memory, but still I only get that single red light and an 88% complete. If I only had some way to even see the output of that last validator run, I would no doubt see the answer right away. But I got nothing here.

Alas, I’m just guessing now, and I’m out of guesses. Has anyone else encountered this, or am I just doomed to be stuck on 88% for this puzzle?


Okay, I got it. Apparently that last test doesn’t like queues. I tried other data structures and finally hit on a win with a hashtable. (Speed maybe?) Done and done.

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May be your first solution wasn’t efficent enough to handle the last secret validator.

Is it possible to complete this task using Java language? My solution is using the ArrayList and it also failed. How many milliseconds or seconds do I need to pass this last case?
P/s: It took me 26 minutes 47 seconds to pass the last.

Yes, and I have not used any lists in my solution.

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I made it using java. I think the programming language is not a constraint, it is possible to solve it using any language available. 26 min is a lot of time. It takes me milliseconds to solve the last case…

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You doesn’t need to store the river sequence. You need only two integer variable :

in a loop
   compare varA with varB
   try to see if they are equals
   apply river rules if needed
end of loop

print the result

you need only basic variables. The real challenge is in your ability to calculate the next number following the river rules.


Yes, I only need to do the basic steps to finish this task. So silly me to overthink in this case!
Thanks everyone. :smiley:

One of my fatal flaws as a programmer (I can admit this) is that I have a tendency to look for big solutions to small problems. I read the problem description and immediately thought, “hey, I can do this with dueling queues, and clean them up as we go, and yadda yadda yadda…”

And then, once I had checked literally every other element of the program, I was forced to conclude that the queues were the issue. Had to be performance. That’s when I also realized that using a queue there was bringing a gun to a knife fight - too much unnecessary functionality for a simple problem. I made everything simpler and it worked just fine.

Once again, I guess: KISS. Keep It Simple, Stupid.

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A kind reminder: Please search the forum before opening a new post on the same puzzle :wink:

Hello everyone!

I am currently taking a break in the middle of the night because I thought I finally got the solution right for this problem…Only to find out I only pass the first test.
I have performances issues on every other test altough I tried to keep it as simple as can be.

Can anyone help me? That’d be much appreciated!


I used recursion and probably had some stack overflow with the last test. When using a simple loop instead of the recursion, the test was successful.

Any hints on how to optimize the code using python?
I am stuck at the last submission test.
I am using one list for each river which i fill using the same loop. Every iteration, I check if the newly added item in riverA is in the river B or not. Same goes the newly added item to river b.

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Hey @dma092, I wrote that exact same algorithm in C# today, and I fail at the last validator too.
Did you finally pass it?

@dma092 @Eldoir try to print the elements of the river that you compute every turn. Maybe you’ll see that you’re doing a lot of computations for nothing.

Or to compare it with the real world, would you check if a big river in a plain will soon meet a small river in a mountain?

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@TwoSteps Thanks for the tip :slight_smile:
That’s what I thought, before doing anything, I should “upgrade” r1 to r2 if r1 is significantly smaller than r2.
Since the last test, “Even bigger”, showed similar numbers (15485863 and 15215260), I thought that it was probably the same thing for the validator, but apparently it’s not.
I pass it now, thanks !


Hi all. Not sure what is going on with my code BUT it works totally fine for all 9 test cases and it fails during the final submission with the score of 88%. It looks like I have a problem with the last case “Even bigger”. It’s strange because same test works fine when you run all test cases… Can some one help me with this issue?

Hi. Thank you for the pseudo-code. My main loop (using Python3) looks very similar to your logic:

while isMet == False:
    #print("r_1: " + str(r_1) + " r_2: " + str(r_2), file=sys.stderr)
    if r_1 == r_2:
        isMet = True
    elif r_1 in secondArr:
        isMet = True
    elif r_2 in firstArr:
        isMet = True
    elif r_1 > r_2:
        r_2 = calcFollowingValue(r_2)
    elif r_1 < r_2:
        r_1 = calcFollowingValue(r_1)

However, I still can’t figure out how to solve the problem without storing values, I mean we have to track those in order to compare between them…? That is why I am using two lists in my solution…

Nope you doesn’t need to track all the sequence.