Create a Sudoku Solver In Java In 20 Minutes - Full Tutorial

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Let's create a Sudoku solver in Java!
Sudoku is a logic game, where you fill in numbers from 1-9 in a 9x9 grid. In Sudoku, each row, column, and 3x3 sub-grid must contain all numbers from 1-9.
Solving Sudoku in Java seems like it could be complicated - and some of it certainly is! But I'll break down the Java program for solving Sudoku into more simple pieces that we'll code together.
Learn or improve your Java by watching it being coded live!
Hi, I'm John! I'm a Lead Java Software Engineer and I've been in the programming industry for more than a decade. I love sharing what I've learned over the years in a way that's understandable for all levels of Java developers.
Let me know what else you'd like to see!
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[@19joni69]

Making a sudoku solver was actually the very first project I programmed once I'd learned the basics. This takes me back.

[@yahia1355]

Wow ! I have been coding for arround 5 years and could'nt come up with a solution!

[@19joni69]

@@yahia1355 oh I didn't do anything smart. I literally brute forced a solution. Basically going over every single square and trying the first number that works, then if I got to one that had no possible correct number I would go back until the first square that I could have put a different number in and tried the different number. Do that until you can fill every square correctly. Just literally trying every combination until it works. There are much better ways to solve this. But the not so smart solution works.

[@user-cg4vi4kn4v]

@@19joni69 🤣🤣😂

[@clo-1588]

You're an excellent teacher, John. I find your videos not only extremely helpful, but impressively well-made - and it's not even so much about the format (which is ideal), but the way you teach and the examples you give. You explain things in a logical, progressive way, and you're excellent at providing the right level of exposure. I do wish there were more teachers like you in universities - but then, we might not have you on KZbin ;) Please keep going with these tutorials, you are contributing to the community in a huge way.

[@stormybear4986]

This was, legitimately, the most interesting java tutorial I've seen in years! EXCELLENT!!!!!

[@janekk3397]

I'm so glad channels like this one exist. It was a part of my project and I would've never figured that out myself...

[@navyathamarreddy2807]

very nicely explained John, I saw few other videos as well for Sudoku Solver, but this is amazing!

[@krishnawadhwani5393]

I had recently discovered this channel, haha, and I started to watch your videos on TV Like I am watching some movies or some kind of entertainment, nice videos keep it up, subscribed after watching 2-3 videos

[@MykhailoZubanych]

Same)

[@nikhilraov100]

This program works perfectly. What a flawless algorithm you have written .

[@jagi7976]

It’s beautiful when something that’s educational is also entertaining

[@saviobatista9619]

Thank you! Amazing video, where I work I tried several times create something to solve our work schedule with some kind of algorithm but always fail, this was just exactly what I needed and hope to solve my needs. Thank you!

[@olayiwolaakinnagbe676]

This is the best video I found online for this problem. You are a great teacher, thank you.

[@rajeshg3570]

This is excellent .. i simply love the naming conventions of the vars..which makes its easy to read and understand the logic

[@mdrwsh]

it is amazing to see various algorithms that be used to solve sudoku, i made one but using while loop

[@zimbabwe8189]

Great video, your explanation was pretty clear!

[@sigfigronath]

This was really nice, fun and informative!

[@luigiq6429]

Great video John. Congrats on 500 subs!

[@CodingWithJohn]

Thanks!!

[@CodingWithJohn]

Well at least I've certainly gone well past 500

[@abdurrouf4159]

You just nailed it. Mind blowing tutorial, no doubt.

[@Nikolai4567]

I didn't think this solution would have an adequate time complexity. Thanks!

[@carlostitlan]

Your explanaition is soo cool! You even make feel this stuff is easy.

[@abidoyevictor2780]

Well explained! Thanks John

[@devforall2006]

Simply Nailed It, AWESOME EXPLANATION

[@qurdedu4032]

Great explanation and great mini project Thank you so much. Looking for more project like this

[@staceyonuora5329]

Great video, I really enjoyed your explanation. Super thanks, this was very helpful

[@ulyses1018]

Great video! Thanks for this kind of tutorials John! :D

[@penguin7863]

Sir before I watch your video I was a student who was struggling to solve recursion problems for my upcoming test. Now I get the idea about how to backtrack in recursion :) Huge thanks from South Korean student!

[@jhuluan-jyun2594]

No wonder you’re a lead! Very clear sir, thank you

[@Garrison86]

Amazing! Thanks for this, this helps me stay engaged in java

[@michaelkraemerman2009]

This was such a good explanation!!! I feel a lot more confident about implementing this. Also thanks for not acting like this is easy/obvious👏

[@guidopiotrowski7900]

Thanks for the great explanation! Keep it up with the channel!

[@sharadgupta8576]

It was just really awesome and in so nice way u described it's just really wow... M so surprised why just so less likes I have watched couple of videos before but u just explained really understanding with gotcha learn some new concept as well successfully found my final year project as well thanks for making this video... I will surely check out your more videos.. Great job 👍

[@tonyz2203]

OMG, I understand how to solve this complex problem now. Thank you so much!

[@JonasKeil]

This tutorial is excellent John.

[@siddhantanand5751]

Hey, John I loved the way you went forward with explaining the video , you made it quite easy to learn the algorithm and fun too, i was curious if we can attach some image recognition tools like OpenCV etc for Java , so that we can scan a sudoku at run time and give an image back? I would love if you could show us how to do this if you like the idea.

[@francescopiazza4882]

Great coding John! I got a "Hard Sudoku" solved in a few ms !

[@alzaeem79]

Beautifully well explained, thank you sir.

[@mira1m198]

Great explanation, easy to understand, even for beginners :-) Thanks!

[@kingsuley0581]

Loved this!

[@sritamabanerjee1331]

Loved your explanation..keep it up buddy 😊

[@amaralensheriff]

As always, thank you John.

[@jaydoshi5394]

Watching couple of videos from this channel every day in any order. But still makes sense

[@b9944236]

Now I can solve it by myself, thanks a lot.

[@Bury11]

cool project :) i'm fairly new to java, i programmed a small snake'ish game, so i understood the 2d array way better then i thought :D keep up the good work!

[@PathWars]

Awesome, exactly the kind of KZbin channel I wanted: Regularly uploaded coding videos.

[@neeldiyora3575]

Excellent explanation 👏👏👏

[@prade1]

This is exactly what I was looking for to complement my java studies. Projects with step-by-step explained. Please, keep it going, unfortunately here on KZbin, everything seems to be only JavaScript or Python. It would be great more projects with OOP.

[@vivekjj2986]

Thanks so much John. I spent a lot of time searching on youtube for a good tutorial on this. I wish yours was the first video so i could have saved time :(

[@mynameispooop]

Awesome Tutorial

[@estebanlegare]

Really helpful!

[@surajverma-ut4kj]

Just one word for John 🙏🏼 Big ThankYou ❣️

[@Gandobilis]

Very interesting tutorial!

[@Daniel-iy1ed]

That was fantastic. Thanks 🙏

[@joedevenski3291]

Fantastic stuff!

[@AdamantlyAdams]

Bro, thank you for teaching. You are the go to guy!

[@kunalkheeva]

How did you make it so simple, I always appreciate your content. your content is limited on youtube but whenever I get stuck in any problem and you have a video on that, which means, my last destiny is your video. Thank you

[@devoiddude]

Fantastic video and fantastic channel, your videos are great thanks .

[@nezetech]

Dude, thanks a lot!! You saved me

[@AkhtarReviews]

Loved it! 🔥

[@dunayqudretli3382]

Great tutorial !

[@ponder2406]

Wow! That’s so cool! This was an awesome video! Very interesting :)

[@razvanungureanu8897]

I have reached the part where I didn't find a solution to erase the all the numbers of the board and to retry. I know it needed some recursion, but didn't figure out how. Your explanation was excellent and after reviewing your method, everything makes sense. Thanks and keep it up with these kind of videos!

[@wombozombo]

Fun way to learn about recursion 👍

[@nagasivakrishna5660]

wow,next level explanation

[@user-jh9ut7in8i]

like your explain!! very clear!!!!!

[@northnorth9913]

I like what you doing, Hope you get the chance to do Dijkstra’s algorithm, Time complexity and a few more interview related. I like your explanations

[@joekagerer]

I wrote a Sudoku solver in 2008 in Javascript, but it didn't quite use full recursion (my mistake) so it wouldn't solve moderately difficult puzzles. Mine includes character recognition of uploaded websuduko puzzles. This week I added "paste from clipboard" so that I didn't have to save/load my puzzle images. Today, following your model I finally got mine solving even the "evil" puzzles. Thank You John for this great lesson. Although now I don't feel that "I wrote the code", my page works and I did right the optical character recognition parts. I don't believe that I can share a link here, but I can try.

[@josueramirez7247]

In an intro to programming textbook I own, it mentions in the preface that one difference from prior printings is that it removes the sudoku solver example code from the multidimensional arrays chapter because apparently is it too complex at that point.

[@mohammadsadrayeganehfaal2323]

such a enjoyable algorithm!!!

[@footballalliance2412]

Hi John, i have learned a lot from your videos. I hope you won't stop making videos in upcoming days, it is really helpful for us. I can guarantee you will get a huge subscribers in coming days.

[@CodingWithJohn]

Thanks! Glad you're getting something out of them. I'll keep making them if people keep watching them!

[@danielmago4327]

amazing professor!

[@stephanc7192]

You make truly great videos

[@Darya-pu6ik]

its fantastic, i really like the way you teach, its intresting! However, could we do it using DP or is the an NP?

[@shintaromidorima1552]

it is 100% working teacher john and thank you for the source code

[@cptsalazar4986]

Awesome johnny...

[@javawocky]

Very nice. I do this stuff for fun. Recently did one of those word search generators which you see kids books all the time. Way more interesting to do than you may initially think.

[@rams2478]

WOW.. you are amazing....clean.. detailed.. explanation. Please do Leetcode problems also.. It will help us.

[@38YD]

love this!

[@surjansr]

Great explanation. Will be great if we can also talk about the time complexity of the solutions

[@robertoborelli3275]

It's like O(k*10^(k^2 -n)) where k is the number of cells in a row and n is the number of placed numbers at the start. If you try with k greaters than 9 this algorithm would literally take years to find a solution. You can do better using sat solvers and translating the sudoko problem into cnf logical formulas.

[@smrtfasizmu6161]

@@robertoborelli3275 may I ask why do you have zero in the beginning of the expression 0(k * 10^(k^2 - n)) doesn't it just cancel everything to zero? Also, why do you have 10 to the power of cells to solve, how did you get 10? Did you mean k because with every added unsolved cell you need k times more traversing the loops (assuming that the number of rows = the number of digits you can fill a cell in) ? And why do you have k * in the beginning of the expression?

[@robertoborelli3275]

@@smrtfasizmu6161 it's been a while since I've watched the video and now I don't remember exactly the code but here I give you some explanations: 1) It's not a 0, but is a big O. This thing is called "Big O notation". Simplifying, you can read it as "the complexity is less or equal than..." but there are some formal and accurate mathematical definitions behind this intuition I gave. 2) The intuition behind the formula is that this code is clearly exponential in the number of unsolved cells (k^2 - n). 3) each time you call the function to see if a number is valid in cell you spend O(k) time. 4) For each cell there are (exactly) k possible values (in the case of normal sudoku k is 9). So to sum the important thing here are: - exponential in the number of unsolved cells. - you take linear time in k, each time you check if a number is valid So as I wrote in the previous comment this is a pretty easy solution to write and implement but it's very inefficient since it uses the technique 'guess and verify'. It's much better (but also a bit more difficult) to use a sat solver for this kinds of problems.

[@juanmalpartida1333]

Time complexity looks to be O(n^3) worst case. Average, eyeballing it, I would say is n^2*log(n)

[@robertoborelli3275]

​@@juanmalpartida1333 no. just because you have 3 nested loops you can't always say that you have O(n^3). Inside this loop there is a recursion! This solution is exponential for sure, infact there are situations in which you guessed j correct values, than you are trying to fill the j+1-th value and you discover that there is no such a solution. In this case the algorithm does "backtracking" and there are situations in which all the j guessed valuesare wrong and the algorithm must re guess the entire solution. This leads to exponential cost!

[@djneils100]

this is a quality tutorial

[@user-sr7ht8rm6r]

John, thank you for everything! Could you maybe make a video on backtracking specificaly? That would be awesome! :)

[@lukaskock7020]

Thank you very much!

[@manikandanm5567]

This is cool, thanks man

[@strawberrykitty8337]

thank you so much!!!!!

[@obitouchiha7545]

Wow sir you can explain things very well, let aside being multitasker.

[@buddybuythis3889]

Wow! Sooo nice and free! Instant sub and hooked. Binge time.

[@JannisAdmek]

You teaching skills are really impressive, great work! I have one minor point to critique, you hardcoded GRID_SIZE 9 but used 3 as a magic number. You could have computed it once sqrt(GRID_SIZE) or just declared it as a constant.

[@Michael-se7ny]

sqrt(GRID_SIZE) would make no sense at all

[@JannisAdmek]

@@Michael-se7ny Really? Doesn't a Sudoku box always have side length of sqrt(GRID_SIZE)? But I guess the really clean way would be to define the sudoku in terms of the BOX_SIZE, since GRID_SIZE has to be square number. so a normal sudoku is BOX_SIZE = 3. (GRID_SIZE = BOX_SIZE * BOX_SIZE)

[@p_varma17]

@@JannisAdmek right

[@mastershonobi110]

Gr8 vid. Spent the last 2days working on this really breaking it down. I got a lot outta following and really understanding what’s going on here. Keep them coming…🙌🏿🙌🏿

[@techtonight882]

I think we can use dynamic programming too, to reduce some of the complexity. We can use a HashMap to store if a number at particular position was a valid placement or not.

[@kesarscorpio]

i guess this one is for beginners and that's why he chose this way of doing it

[@cs0576]

But that is dependent on the previous placements. So I do not immediately see how that improves this algorithm.

[@smrtfasizmu6161]

The argument of the hashmap needs to be both the board and the placement of the number. How does backtracking in your algorithm work? I am not sure how you intended to do it with hashmaps and make the code less complex than what you saw in this video. Can you elaborate further? One idea popped up in my mind, but it is more or less a copy of what you saw in the video but without recursion. First you have an array which holds the position which are given in the beginning. Then, you proceed filling in the board and every time you find out that you can't continue filling in the board you backtrack the i and j variables of the loop until you reach the next number which you filled in and continue trying filling it after setting number To Try to be 1+the previous numberToTry.

[@mr.mirror1213]

iirc the whole problem is a system of linear equation , and u can solve it in O(n) if u use multigrid method

[@sanchitghai1455]

I like that you also play binding of issac 🙂. As for the program I loved it. It was well made video for beginners and one can't not understand the concept. Keep it up! 👍

[@samuelnilsson5505]

Love this

[@Pennervomland]

I‘d love to see this exact algorithm but more efficient. Knuths Algorithm X for sudoku sounds very interesting and shows a pretty good method for backtracking. I‘m too dumb to understand how to implement it in java but it could be a cool challenge for you.

[@Kanha0321]

Its amazing ❤️

[@GeorgeVrachliotis]

Brilliant!

[@user-ou1ir7zq8o]

I learn more in 1 video than in 2 months worth of other tutorials

[@abubakaradamu6278]

I think, i just found my best youtuber. Keep up the good work John.

[@patrickbradley4948]

Same

[@RomualdBrunet]

Using static methods and passing the board as parameter hurts my brain for some reason (vs using the board as an instance of a class)

[@ae6072]

same! Why doesnt he use the board as an instance? Is there any special reason for this?

[@manusoftar]

I think that those nested for loops on the solve method are unecesary, you could add two additional params to send the current row and column and inside the function you just would need to either add the the rows or to the column depending on what you are doing... yes you will have to put some IF's, but the way you did it I think it will do unnecesary iterations, I mean, the last recursive call on a solvable board will put the needed number but there's nothing that would stop the initial call to the solve method from stopping it's iterations. Actually, on each recursive call it will have to traverse as far as possible on the board until sending back a false or a true if solved. You don't need the algorithm to traverse almost the whole board on each recursive call, you just need it to make a single step on the board instead.

[@jamesdepoorter726]

Hello John, why didn't you use TDD in this example? This could be a nice addition, because it constantly keeps the code in check

[@bartomiejpotaman6973]

Shit took me a while but I think that's why it was worth it. This video made recursion seem so much more intuitive. Thank you!

[@pratikdhargalkar4876]

Subscribed !!

[@verruu333]

if you could make a short tutorial about binary search trees and how to understand recursion there..

[@alpaslanbek]

It violated every clean code principle I know :D