4 Sum | Brute - Better - Optimal with Codes

Problem Link: bit.ly/3It5SyP
Notes/C++/Java/Python codes: takeuforward.org/data-structu...
We have solved the problem, and we have gone from brute force and ended with the most optimal solution. Every approach's code has been written in the video itself. Also, we have covered the algorithm with intuition.
Full Course: bit.ly/tufA2ZYt
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0:00 Introduction of Course
00:54 Problem Statement
02:24 Brute force approach
03:08 Code
05:38 Complexity
06:14 Better approach
11:31 Code
13:22 Complexity
14:54 Optimal approach
21:59 Code
27:04 Complexity

[@takeUforward]

Please watch our new video on the same topic: kzbin.info/www/bejne/m3WcZoqIm81rZ8U

[@uniqueyuvrajgaming3392]

You said it is O(n3) but sorting will take some time to what about that?

[@vaibhavagarwal1479]

​@@uniqueyuvrajgaming3392nlog n is basically negligible in comparision of n3 thats why is it not included

[@user-hb2ex3wg9k]

Sir,I have one confusion about the time complexity. Inside the innermost loop we have the insert operation in set which take O(logN) in the worst case considering we have n elements inside the side. If we are taking the worst case means that the if(sum==target) is true for each condition.Then Why we are not considering it's complexity. According to my calculation,Time Complexity is O(N^3(logN)). Please sir reply. Thank you in Advance.

[@user-hb2ex3wg9k]

@@uniqueyuvrajgaming3392 Sorting will have always 4 elements which will take constant time.

[@shailesh_rajpurohit]

Note: 0:42 Make sure you watch the 3Sum before doing the 4Sum . 🙂

[@TrashAccount-ye3ul]

What if I have seen since a long ago, will it work ?🙃 But why he was laughing while saying this ?

[@nikhilKumar-xv4mt]

😂😂

[@kanhiyasharma3648]

naughty audience😂😂😂😂

[@MandeepSingh-rr8kk]

ok 🙂

[@shubhrajyotipoddar1684]

bro couldn't control his laugh during 3sum , 4sum intro and had to cut the clip 🤣

[@yashsharma6396]

Excited to do 4sum after completing 3sum yesterday

[@gareer2436]

Damn bro include me next time

[@yashsharma6396]

​@@gareer2436 you know how to bend?😂😂

[@gregorirasputin659]

Yaar tum log bohat tharki ho 😂

[@gareer2436]

@@yashsharma6396 no but I know what to do when someone bends...😏 Lay them down and.. Massage their backs of course!

[@gareer2436]

@@gregorirasputin659 ma to sath me problem solve krne ki baat kr raha tha bro... tum kya soch liye?😏

[@ashishashish1403]

Whole video is about 4 sum but 0:40 to 0:50 is wholesome

[@jatinsareen7771]

3 sum was awesome, 4 sum was fantastic!!!!

[@suyashrahatekar4964]

legends like 5 sum

[@takeUforward]

Do give us a like and subscribe, it won't cost you anything, but it will highly motivate us to create more such type of videos :)

[@rishav144]

striver ..king of DSA

[@shra1]

The if condition inside while to check if(k

[@codemania2878]

sir which whiteboard pen app you use for explain the code

[@itsmrcrazy7670]

hey striver you forgot to add time complexity required for --sorting array --optimal approach so it will be O(NlogN) + O(N^2). plz update

[@user-lz1rv3em5u]

@@codemania2878 He is using an iPad and its default Notes app​

[@impalash_ag]

Hi Raj, there are 2 slight mistakes in your optimal solution. 1: The 1st for loop will run till n-3 and 2nd for loop will run till n-2 instead of n, because we need to allocate last 2 elements to our two pointers and as per your code when i=n-2, j becomes n-1 (j=i+1=n-2+1) and num[k] throws out of bound exception, because k becomes n(k=j+1) 2: We could also increment low and decrement high when sumtarget respectively until there are duplicates for code to be more optimised. 3: Here's the more readable code(JAVA): class Solution { public List fourSum(int[] nums, int target) { int n = nums.length; List result = new ArrayList(); Arrays.sort(nums); for(int i=0; i

[@_Arunvfx]

hatsoff bro i was confused for 4 hours for optimal solution that passes all test cases.after seeing your comment it helped me to understand it.thankyou :)

[@impalash_ag]

@@_Arunvfx glad you finally understood it. Keep pushing forward 😃

[@Akash-yr2if]

0:44 Striver's Smile Says it all

[@brijmohan_f_s]

you got my point bro😁

[@AkOp-bf9vm]

bhai bhai🫡🫡😂. what a detailing bro

[@selvaarumugam370]

the way you explain the two pointer approaches and the use of them makes two pointer approach the easiest way to solve a problem. I fell in love the way you are teaching these concepts.

[@arujgarg7267]

Finally did 4 sum, amazing experience

[@gareer2436]

Can you elaborate your experience? Asking for a friend

[@catalyst8654]

can you explain about your experience...🙂🙂

[@leisuretime4198]

@@catalyst8654 🤣🤣

[@Akash-yr2if]

Would you take a moment to share your experience with us.

[@avinashkumar2411]

You should not take I upto

[@mehulthuletiya497]

00:54 Problem Statement 02:24 Brute force approach 03:08 Code 05:38 Complexity 06:14 Better approach 11:31 Code 13:22 Complexity 14:54 Optimal approach 21:59 Code 27:04 Complexity

[@takeUforward]

You watch all the videos? I see you daily updating me with timestamps, massive thanks for that.

[@mehulthuletiya497]

​​@@takeUforward Yes, I watch the all the videos. After the watching video i comment the Timestamp. Recursion and Backtracking series is the one of the best explanation and understanding with your teaching style. I completed 2 days ago.

[@salihedneer8975]

@@takeUforward striver please update timestamp in disucssion so that ppl can access it easily

[@hareshnayak7302]

Understood,Thanks striver for this amazing video.

[@dishankbaid05]

The consistency level of striver is really the source of motivation 🔥.

[@praveennemagoudar3442]

wow,....wow....wah..awesome expaination. undestood bro..hatsoff to you.

[@kanyapandey]

Thanks a ton for explaining in simplest way 🙏🙏🙏🙏🙏🙏

[@ra-onegaming1383]

I have done this question using subsequence technique thanks u raj sir 😊 u teach really simple

[@nquanta1548]

Thankyou for the wonderful series ❤️🔥

[@bkmeher9005]

uffffffff.... understood , bahut bhala se bujhi hela bhai

[@helloworld2054]

This has to be the horniest coding problem 🌚

[@cinime]

Understood! Super wonderful explanation as always, thank you so so much for your effort!!

[@AbhishekKumar-cv1dh]

Understood beautifully!! 🔥❤

[@joeljacob4685]

Understood!! Thanks for the nice explanation

[@footballcreativeeverywhere260]

Great bhaiya ❤️🇮🇳 , ready for next one

[@AniketKumar-hf2bo]

understood,thnx for the amazing video ❤❤❤❤👌👌💕💕

[@adebisisheriff159]

Striver..... you are amazing.................................... Thanks so much bro

[@shivanichaudhary1427]

Amazing, thankyou!

[@sarangkumarsingh7901]

Awesome Lecture Sir............

[@MdKaif-bf3nz]

Thank you so much Striver brother, you taught the 3sum and 4sum very clearly. 💀💀 ok jokes apart lol, this series is just too good, i can literally find every concept DSA related a lot of questions of each and every concept.

[@gauravsoni5424]

best explanation on you tube

[@akankshajain3997]

helpful to understand integer overflow

[@bikramprajapati5358]

Amazing Explanation

[@DR-mq1le]

understood , thank you!

[@konankikeerthi]

Thank you bro. Understood

[@satheeshbhukya9970]

excellent question....

[@UECAshutoshKumar]

Thank you!

[@raghavkansal3765]

you are the best 🧿🧿

[@senseiAree]

Understood ❤

[@AbhimanyuKumar-pw3je]

if u are stuck at test case 281 in leet code then :- class Solution { public: vector fourSum(vector& nums, int target) { int n =nums.size(); vector ans; sort(nums.begin(),nums.end()); for(auto it:nums) cout

[@lakshmiprasanna7058]

Understood 💯💯💯

[@Shivi32590]

thank you

[@subhamcoder]

Sir app apne resume k project me konsa tech stack use kya haii. ❤ love your videos

[@user-sm7zo5zd9t]

Understood🤙

[@ravitejachandana6199]

understood !

[@simonchristopherp3260]

Understood!

[@hiteshpanchal5772]

Thxu striver bhai 😊😊

[@rajatpatra2593]

Understood ❤️

[@per.seus._]

understood ❤

[@THUNDER-kw3wq]

Striverr bhai ke liye toh 10some bhi asaan hai...Jokes apart hes the best teacher on youtube

[@user-nb1ye5tf1r]

Nice to do 4 sum.

[@dhairyashiil]

Thank You : )

[@her_soulmate]

Understood 🎉

[@VishalPanwar-df5ek]

Understood Srtiver!

[@hemantpatel1413]

Understood.

[@himaniupadhyay8201]

Thanks Lord

[@Str13011]

Keep going 👍

[@sanayaagarwal1907]

understood👍

[@mdazizul-y7b]

understood sir

[@rishabgupta2733]

The complexity of better solution is n^4 *logn according to codestudio

[@ayanakash9900]

For optimal solution time complexity, we have to add sort() time too ig

[@RohitKumar-dy2gc]

easy peasy✨✨

[@culeforever5408]

understood ;)

[@user-nk1mb5fy7j]

UNDERSTOOD

[@ankitadas5833]

Though I am little bit late, in this A to Z DSA Playlist But I covered All previous Videos within a week & now just completed 4 Sum. Striver Sir You are great! You will going to create a history in DSA course very soon around all over the World … I don't have the proper words to express my gratitude to you. All the Best Sir God bless you .

[@ankitadas5833]

@@ArjunS-hi3vl Yes

[@DR-mq1le]

have you completed the course ? im also quite late in starting the course and moving at about the same pace as you

[@ankitsharda1131]

@ankitadas5833 hey I would like to know if you have completed entire striver a2z dsa course? If yes, how much time did you take to complete entire course?

[@jayanthvanamamala1425]

Bhaiya , Can uh please post an article about k sum similiar to this type , which is applicable for all k sums . It can help to build intution!!!!

[@user-st9ff2vh6l]

Understood

[@soumiyamuthuraj3516]

Awesome

[@asgarh4589]

People who are aware of 2 pointer approach, watch from 14:56, saves a lot of time.

[@VishalGupta-xw2rp]

After doing 4 Sum.... I think Sky is the limit 🌝

[@omkarsawant9267]

Time Complexity: O(n log n) Space Complexity: O(n) #include #include #include using namespace std; vector fourSum(vector &nums, int target) { // sort the array sort(nums.begin(), nums.end()); // initialize result vector vector result; int n = nums.size(); // iterate through array with two pointers for (int i = 0; i < n; i++) { // skip duplicate elements if (i > 0 && nums[i] == nums[i - 1]) { continue; } for (int j = i + 1; j < n - 2; j++) { // skip duplicate elements if (j > i + 1 && nums[j] == nums[j - 1]) { continue; } // use two pointers for remaining elements int left = j + 1; int right = n - 1; while (left < right) { int currentSum = nums[i] + nums[j] + nums[left] + nums[right]; if (currentSum == target) { result.push_back({nums[i], nums[j], nums[left], nums[right]}); // skip duplicate elements while (left < right && nums[left] == nums[left + 1]) { left++; } while (left < right && nums[right] == nums[right - 1]) { right--; } // Move pointers left++; right--; } else if (currentSum < target) { // If the sum is less than the target, move the left pointer to the right left++; } else { // If the sum is greater than the target, move the right pointer to the left right--; } } } } return result; } int main() { // Example usage vector nums = {1, 0, -1, 0, -2, 2}; int target = 0; // Call the fourSum function vector result = fourSum(nums, target); // Print the unique quadruplets for (const auto &quadruplet : result) { cout

[@akashchoudhary6876]

kuch bhi, same solution hai upar wala time complexity n3 hai, apki nlogn kaise aa gayi

[@omkarsawant9267]

@@akashchoudhary6876 the overall time complexity of the fourSum function is O(nlog⁡n+n^3)which simplifies to O(n^3)in Big O notation, assuming the input is sufficiently large.

[@omkarsawant9267]

@@akashchoudhary6876 sorting in cpp will take O(nlogn)

[@203-aasritha2]

We should consider the time complexity for sorting the given array right ?

[@myaccount09011975]

What about Time complexity for sorting the arrray? I know it will be lesser than N3 in this case. But still good to mention that!. Thanks for putting all the hard word!.

[@tanishagarwal5992]

When striver submits on Code Studio , it also showed him time complexity as O(n4 log n) but it doesn't show in mine. Why?

[@tejaspatel2212]

Imagine someone getting your video, on Searching 3sum video or 4sum video on google 🌚😂

[@shis10]

Optimal Orgasm 🤣

[@himanshukaushik9223]

😂😂

[@SiddheshKukade]

thanks

[@vagabondfoodie5788]

Aaj maza aa gya 3 sum karke🌚🌚🌚 ab 4 sum krunga

[@AbjSir]

understood

[@gregorirasputin659]

Nice explanation. But the title is a bit too sus don't you think?

[@iamnoob7593]

US , THANK U

[@AkshayYadaveeb]

while i'm taking sum==target case in else it is not working like we did in three sum and if i put if(sum==target) at initial position it is working . why it is so ?

[@chathuryachenchupalli6535]

Hi sir We request you Can you please make a video on Alex goes Shopping problem. We cannot understand the question.

[@syedFAHIM-el1wr]

Under stood

[@thomasshelby6780]

Tapped into pleasure back to back with 4Sum followed by 3Sum

[@Manishgupta200]

It's amazing from brute to better to optimal similarly like 2Sum, 3Sum .. and my doubts in what's about 5-Sum, 6-Sum.. is any universal solution exist?

[@harshshukla7675]

Yes, the complexity will keep on increasing tho

[@srikarvinjamara2003]

Hi, thanks for the video explanation. I had one doubt. When we get the sum==target, we move the k and l pointers front and back respectively [not just once but until we get a different value for k and l pointers so as to not repeat the same ans]. Why are we not doing the same when {sumtarget}. [i.e. in these 2 cases we just do k+=1 or l-=1; shouldn't we here also move the k and l pointers ahead until we get a different value for them.] Please let me know if possible. Thanks!

[@203-aasritha2]

It goes the same as well. If we move the pointer and again we get the same set , the sum will be the same right ? therefor if the sum is less than zero , it remains the same and vice versa. Then the iteration completes and we will hit the same condition. This goes until we get another pair.

[@satyamsrivastava3423]

Just wanted to add the Time Complexity will also contain log(n) since we are sorting the array, right?

[@many987]

understood.

[@himanshukaushik9223]

Bhaiya is everything fine any health issues video is not coming ??

[@arunmahajan1028]

After Understanding this concept , I can Say that Sky is the limit..

[@user-mk3jx1bs2g]

in the brute force approach when we first added nums[i]+nums[k]+nums[k]+nums[l] and then checked if its == target then it gives integer overflow error regardless of sum being stored in long long variable but if we do it one by one like sum = nums[i]+nums[j] and then sum+=nums[k] and sum+=nums[l] then the error is resolved can anyone please explain to me how its working.

[@EC20022ELAKKIYAC]

did you found how does it works ? same doubt here

[@hydrocy.9165]

23:16 shouldnt j= i+1 will be second element? I cant understand

[@D3vansh]

I think first i should learn 3SUM then will do 4SUM as you said

[@takeUforward]

Please learn 2 sum, then 3 sum, then only 4 sum should be implemented ! Yes in terms of problem.

[@gandhamphotos]

@@takeUforward 😂😂

[@shis10]

@@takeUforward 🤣🤣🤣

[@godson200]

@@takeUforward sir but partner is not available how to do 2sum

[@mohsin7343]

I am still with 1sum

[@VivekKumar-kx2zf]

why we are putting condition while(k

[@brp3522]

If k crosses l then we have gone through all the possibilities. Imagine this if they crosses each other they will act like each other. After crossing K will give the elements L has already given and the same goes for L

[@Sachin_mry]

bhiya unique no of occurences ka que bina set and hashmap ke kara dijiye. ekdum samjh nahi aata ye que

[@anupamkhare7041]

is this the end..or more videos will come....I'm about to start this course.

[@aysams2]

04:20 - what does exceeding the integer limit mean? (when we are storing it in a long long)

[@takeUforward]

if you store in int it might exceed

[@SANDEEPKUMAR-in6li]

@@takeUforward but in 3 sum we sum up like sum = nums[i]+nums[j]+nums[k] but in this 4 sum you sum up all the indices separately which is correct but I am getting error in a test case when I am doing sum = nums[i]+ nums[j]+ nums[k]+ nums[l]; why?

[@Monika-vs6bw]

@@SANDEEPKUMAR-in6li the compiler has an algorithm for adding these numbers . while adding four numbers(big numbers) the algorithm needs a lot of space , but while adding two or three the space is under limit. hence adding them one by one is recomended.

[@AmanKumar-qz4jz]

bhaiya greedy par playlist bana do

[@saurabhtamta7295]

why is it when i use:- long long sum = nums[i]+nums[j]+nums[k]+nums[l] -----it says runtime error but when i use :- long long sum = nums[i]; sum+=nums[j]; sum+=nums[k]; sum+=nums[l]; ----it works fine. Why is that?

[@anujraut426]

Gr8