Subarray Sum Equals K - Prefix Sums - Leetcode 560 - Python

[@beeramrahulreddy11]

u forgot to add 2,1 to the hashmap

[@TobiasWheaton]

Good catch, i was thinking about that too bc its easier to see the pattern if its consistent on when u add (even if its not needed for this example)

[@prasad9012]

I just spent like 10 minutes rewinding and forwarding the video trying to understand why 2 wasn't added. Thankfully, I found your comment now.

[@dopark1027]

Actually, it is just not {2:1}. He forgot to add the current sum to the hash table when it is not found in the table.

[@amritkumar8876]

Yup .. I was also thinking about that ..

[@pwnweb5734]

haha yeep. my OCD went brrrr... I was like wheres the 2 :P ..

[@suvajitchakrabarty]

If you are having difficulty understanding this or wrapping your head around the solution, maybe this might be a good way to think about it. The question asks how many subarray sum equals to k. For a subarray to sum to k, you need a subarray, as in a part of the array from index 'a' to index 'b' to have a sum equal to k. But when we reach any index 'b', we obviously do not know, if there was a subarray from index 'a' to index 'b' equal to k. However we do have the sums from 0 to index 'a' in our hash map, because we have been storing all sums starting from index '0' to every single index till now, and the count of them as the value of the key. Now obviously sum_0_to_a + sum_a_to_b = total sum so far (curSum). If we go to the original ask, which is we need a prefix that is sum_a_to_b to be equal to k. For that to hold true, replace sum_a_to_b with 'k'. Hence, sum_0_to_a + k = curSum. Hence curSum - k = sum_0_to_a. And then since we have been storing all possible values of sum_0_to_a so far in the hashmap, curSum - k must exist in the hashmap as a key, and we can simply add the value from the hashmap to add number of prefixes from 0 to any index which equalled to curSum - k .

[@meraki___]

Bro this comment is better than the video lmao. Thanks a lot for the explanation.

[@dhairyakhara783]

You made the problem 1000 times easier to understand. Thank you sir!

[@dishade7094]

Thanks for the perfect summary. Awesome bro !

[@kimstuart7989]

You know I remember first learning CompSci, I thought the specific algorithm concepts (BFS, DFS, Dijkstra, etc.) were the hard ones to learn. Now that I am more experiences, I realize that the what I thought then were 'basic' data structures, i.e. arrays, strings, etc. are actually the harder ones to make sure you understand because those concepts has many dedicated algorithms to them, such as Kadane, prefix/suffix array, KMP etc. that you need to know to truly understand that data structure. So while I glossed over arrays because I knew their functions, I am now revisiting to make sure I understand the algorithms embedded in them. Learning truly never does stop!

[@fatcat22able]

Honestly same here. I figured that arrays would be relatively simple, and they are simple on the surface, but there are so many algorithms and different types of problems that involve them that it feels like they're an entire rabbit hole unto themselves.

[@servantofthelord8147]

Facts man

[@davidnwaekwu9019]

factssss, at a point i thought smth is wrong with the way i think .

[@Question418]

I like your attempt to explain it, even though I don't fully understand it. I appreciate it.

[@codetrooper9279]

U should

[@ToastFrench24]

lmao

[@apoorvwatsky]

1:55 I needed to hear that, I wanted to know why sliding window won't guarantee correct answer for these constraints. That cleared it up.

[@EchoZeeDot]

Thanks, NeetCode for these solutions and explanations, It would be really helpful to make a video about general techniques that you use to solve these problems... Because we wan to know how to approcah these kinds of problems by ourselves. Thanks again.

[@mangalegends]

I'm still trying to wrap my head around this EDIT: Came back a few weeks later and it makes 100% sense to me now

[@OusmanBah-f8v]

Me right here

[@josembass]

Great explanation. How are we supposed to come up with a solution like this in 30m without having seen it before!

[@darshana5254]

We cannot. These are classic problems asked during interviews. Whoever come across this problem would easily solve it. Other would go with bruteforce approach which doesn't work on interviews.

[@scionOfIkshvaku1]

@@darshana5254 thanks bro, you’ve raised a very important point. Only if one has encountered such problems, only then will he be able to solve the problem cuz he knows how to optimum approach.

[@mobaisi13]

That's the neat part. You don't.

[@vineethsai1575]

I got this question in the interview and I did not know the optimal solution. However I was able to produce the n^2 solution quickly and the interviewer started giving me hints to use Hashmap to reduce it to O(N). I scrambled here and there but was able to talk about the "repeated work" being done and how I could use hashmap to save the work. In the end I was not able to code it up. Interviewer still passed me though.

[@calebmitcler8760]

@@vineethsai1575 what company

[@scottloewke5199]

Good explanation and, fortunately, the omission of the [key = 2, count = 1] map entry did not affect the result. However, it would be good if NeetCode could add text at that part in the video to make it clear what happened.

[@AndreiSokolov-k7j]

It's not about a bug in the code, it's about a bug in the video. 9:55 Here you should have added key 2 and value 1 to the hashmap and that's it.

[@CliffordFajardo]

Initially I thought it was a sliding window problem, Trying to wrap my head around this ...patience, time...ahhh. Thanks for the video!

[@ziomalZparafii]

I thought about it as a sliding window problem until I found out there can be negative values. If we would only have positive ones, you would increment right pointer while sum is less than k, then increment left while sume is more than k. That would be my approach (not tested).

[@aleksandr_t]

@@ziomalZparafii the main problem behind sliding window approach is some edge cases. For example: [1,-1,0], k = 0 If we will use two pointers approach (or dynamic sliding window), then we will return 2 instead of 3. 1) sum=1, count=0, left=0, right=0 2) sum=0, count=1, left=1, right=1 3) sum=0, count=2, left=2, right=2

[@ziomalZparafii]

@@aleksandr_t you have a negative value in your array. Read my comment once again :-)

[@matom2473]

in a real interview we can not make this mistake, it will take you 20 mins to find out sliding window will not work

[@RuslanZinovyev]

@@ziomalZparafii you still can use a sliding window with negative numbers, it depends on requirements and edge cases, so your first comment is not entirely accurate.

[@timzhang6651]

Well explained, and it takes effort to make a video as good as this. Thank you!

[@darkwhite2247]

08:21 - i have no idea how we are supposed to figure out during the coding interview that (0,1) has to be added into the hashmap by default. How is anyone solving this without memorizing during interviews? May be i'm dumb to not understand this even though the instructor is explaining it. Please help me understand the intution behind adding (0,1) in hashmap by default.

[@AndreiSokolov-k7j]

bcs if you find that n-k = 0 means that you dont need to remove anything so basically it be 0, 1 by default

[@arunraman6630]

A slightly cleaner solution using a defaultdict class Solution: def subarraySum(self, nums: List[int], k: int) -> int: prefixSums = defaultdict(int, { 0 : 1 }) curSum = 0 res = 0 for n in nums: curSum += n res += prefixSums[curSum - k] prefixSums[curSum] += 1 return res

[@user-wx8hn3ng6b]

It's a good combination of two sum and prefix sum. And the intuition should be: 1. subarray sum -> prefix 2. equals k -> two sum The only difference is it's 'b - a = k' instead of 'b + a = k', and you just need to calculate the 'a' differently.

[@Adam-tz6gk]

This is by far the best explanation in the web for this problem, thank you so much NeetCode!

[@edwardteach2]

Another way to think about why we would set our key value to {0:1} instead of memorizing, is to think what if that single element, itself is the target. For example: input = [1, 1, 4] , k = 4 occur = { 0: 1 } By just eyeballing, you'll see the output is 1, since the last element is a 4. When we iterate through the input, we'll do input[2] - k (4 - 4) = 0. Do we have a 0 in our hashmap/dictionary? Yes we do, it's value is 1.

[@alvjou]

except in this scenario wouldn't we be doing currSum - k (6 - 2) = 2, and we would be looking up the value for 2 in our hash map, which would be 1. I think your scenario only works if the single element that is the target is the first element in the input.

[@X3Maverick]

00:40 The brute force approach is O(n^3), not O(n^2). There are ~n^2 subarrays but summing a subarray is O(n).

[@spacetime_wanderer]

It is n^3 if you calculate sum from 0 to n everytime. It is n^2 if you maintain a sum variable for the innerloop and simply add num[i] to it.

[@Wes-Tyler]

You explained it so perfectly that I didn't even need to see the code to be able to code it myself :) thank you!!

[@uzzwalpreetkaur3374]

I understood this explanation! Thanks!! I was trying with sliding window technique first and getting the wrong answer until I realised that we can have negative numbers. This is indeed is a neat.

[@tempregex8520]

i appreciated you explaining this. But, it would have been more helpful(esp to people like me) where you would have taken the time to explain more about the use of HashMap for maintaining prefixSum -> Count to me it felt like the explanation went "too fast" as soon as you discussed the brute force solution.But, in any case a great attempt indeed.

[@unwarysage05w32]

This is a way better explanation than leetcode premium’s explanation thank you!

[@smonkey001]

Godlike explanation to a looked easy problem. Hats off my man

[@dharmvtiwari6286]

There is another edge case for adding 0->1in the starting , which is if the. nums[0] is 0. This intuition is quite tricky but great explanation.

[@roman-berezkin]

Absolutely love this guy, not lessons but a masterpiece!

[@daylansit4537]

By far the best explanation to this problem

[@quirkyquester]

just gotta keep practicing! Thank you Neetcode for making all these amazing videos!

[@cranos666]

Crazy good explanation. Subscribed. Thank you man !

[@lakcai4063]

This is probably the best explanation of this question on KZbin

[@mukeshmanral1327]

what a great explanation , saw many videos, amazing explanation of intuition.

[@benk2451]

Very well explained! Thank you so much!

[@almasabdrazak5089]

so what you are saying is we have a map that shows how many ways we can come up with given sum so for sum 0 we can come up with 2 sequences, but these sequences have to be contiguous , how to prove that 0 has 2 only if we pop up contiguous elements from current sum ?

[@MP-ny3ep]

Hands down the best coding youtube channel

[@Thewowhahahawow]

What a god of prefix sums. Great vid!

[@soanonso]

Really good explanation. Thanks.

[@chenhaibin2010]

brilliant trick of adding a prefix 0 to count no prefix situation instead of another if statement

[@Mohith7548]

I don't know why it works, but still works! ;) Everytime I have to revisit the solution before interview. ;(

[@SonGoku-lc1sb]

Perfect explaination. The best one on youtube so far

[@sgetti4dinner]

I see some confusion on why we initialize prefixSum = {0:1}, and that's because we can always produce 0 by taking no elements from our array (e.g. []). This works out nicely when we run into a subarray that has a sum == k, because sum - k = 0. Then the question we ask at this moment is "is there a prefix-subarray I can take away from my current subarray sum that also equals 0?" The answer is yes, because I can takeaway no elements from my current subarray and the current subarray sum will still equal k.

[@emekaobasi4765]

why do we add to the hashMap after checking for diff? Can we add to the hashMap after checking for diff?

[@jameszhang9422]

Very clear explanation! Even better on the second watch lol.

[@thndesmondsaid]

the idea that we're expected to come up with this solution on the fly in an interview is bonkers to me

[@meylyssa3666]

thank you so much for you clear explanations!

[@Nahwka]

Believe it or not this was the entire solution that actually killed me 😂

[@nivethanyogarajah1493]

Very nice explanation!

[@hwang1607]

great solution thank you

[@celinlu7274]

Your video is much better than the video in leetcode solution. Thank you.

[@GdeVseSvobodnyeNiki]

Finally a great explanation what hashmap actually represents! Got the essentials of the algorithm on the first watch of this video, but was banging my head against the wall trying to figure it out on leetcode.

[@itspete2444]

Why are array and string problems 10x harder than graph, matrix and dp???

[@kaushik.aryan04]

yeah atleast graph matrix dp follow a single type of questions these are like so different from each other

[@phebos2228]

Bless you, O savior from heavens above!!

[@edwardteach2]

My python implementation: class Solution(object): def subarraySum(self, nums, k): """ :type nums: List[int] :type k: int :rtype: int """ occur = collections.defaultdict(int) occur[0] = 1 count = 0 cur_sum = 0 for n in nums: cur_sum += n if cur_sum - k in occur: count += occur[cur_sum-k] occur[cur_sum] += 1 return count

[@michelleyang1881]

really clear explanation. Thank you!

[@zj82119]

nice explanation with good diagram. really helpful.

[@fengliu975]

Did this problem but used an array to keep track of the running totals then reversed it... used a deque after but wow is deque slow and memory intensive

[@xmnemonic]

Added more comments, explanation, changed diff to prevSum which I think makes it easier to understand: class Solution: def subarraySum(self, nums: List[int], k: int) -> int: kCount = 0 curSum = 0 prefixSums = {0:1} for n in nums: curSum += n # We are at curSum, but we want to know how many sum = k # have occurred. k = curSum - prevSum, use the prefixSums # hashmap to determine how many prevSums occurred. Each # one represents a potential subarray that could be # subtracted to reach sum = k. prevSum = curSum - k kCount += prefixSums.get(prevSum, 0) # Update prefixSums AFTER we have gotten the kCount for # this curSum. We ensure that kCount considers only # prevSums prior to this iteration. Including this sum # potentially doublecounts the n of this iteration, # using the n in both curSum and prevSum. prefixSums[curSum] = 1 + prefixSums.get(curSum,0) return kCount

[@MinhNguyen-lz1pg]

Great Solution! Just want to leave a comment and a slight change in hashmap. If you don't want to have a base case, just make sure to add 1 to the count when you see curr_sum == k class Solution: def subarraySum(self, nums: List[int], k: int) -> int: curr_sum = 0 count = 0 sum_to_count = collections.defaultdict(int) for n in nums: curr_sum += n if curr_sum == k: count += 1 count += sum_to_count[curr_sum - k] sum_to_count[curr_sum] += 1 return count

[@АлександрПетровский-м3з]

Very nice & clear. TY!

[@thegod3500]

Finally some decent explanation of this magic

[@kirillzlobin7135]

Can it be considered as a dynamic programming solution?

[@dipeshbhardwaj1747]

Hi @NeetCode. I am not able to understand the part where you say. we need to add the prefix sum to Hashmap simultaneously. even if are maintaining the Hashmap first. we can still get count of prefix - k and prefix = k.

[@anandmehrotra6170]

Take this example: [1]; k = 0. Without adding first your prefix sum array would be: 0->1 and 1->1. Now you will compute prefixSum-k for each key. 0 - 0 = 0, You say you have a match(which is just the empty prefix). Now you compute 1- 0 = 1, you check your map and the value exists, so you return a count of 2. That is why the update needs to happen simultaneously. So, that you can check the subarray between some starting index "i" and current index "k"(excluding k). Hope this makes sense.

[@ShivangiSingh-wc3gk]

Thank you so much!! Amazing walkthrough

[@Obligedcartoon]

This is a LC hard imo, gawd damn!

[@NeetCode]

Agree, it's definitely more difficult than some actual hard problems.

[@mikhassik]

Neetcode's videos are usually the ones that drive it home

[@hoatruong2474]

Very informative man. Keep up the good work.

[@razisyed9481]

Amazing explanation--thank you!

[@lakshyadhawan800]

nice explaination

[@AmolGautam]

Thank you so much for this.

[@slayerzerg]

get() is unnecessary can just add to hashmap regularly. this drawing analysis is goated though!!!

[@anastasiiafilippova5212]

Brute force is n**3 not n**2 as stated. n**2 for all possible subarrays and n for the sum

[@vinethasuresh3488]

Thanks for this video sir , it helped a lot.

[@NeetCode]

Happy to help!

[@odelyaholiday9519]

It's easier to use defaultdict instead of the .get

[@divyanshmishra5121]

I have a doubt. If the array is 6,4,3 and k is 9. we find that sum till n is 13 and 13-k ie 13-9=4 is present in the array so that will get counted but in reality it is wrong as it is not a subarray is we dont pick contiguous element. please explain

[@ngndnd]

i think im just dumb af bc i still dont understand it

[@definitesquare]

Thank you neet bhai

[@orellavie6233]

I am not sure, but take an example where the array is [-1,1,-1] and k>0(e.g., 2). The result would be 1, since after the 1 is covered, and then we went into the -1 again, the result will be 1, and that's not true (should be 0). Am I missing something? I think the results should be added only if the cursum is >=k

[@itachijonin]

really good explanation. my one nit is that you should make your code more pythonic (camel-casing?!)

[@vatsalsharma5791]

Awesome explanation❤️ Can you plz solve Count submatrices with all ones?

[@cbverma2k]

Super like and well explained

[@m04d10y1996]

Why did you initialised your dictionary before hand with (0:1)

[@arifinrahman6]

amazing explanation :)

[@siriussem2]

yey first real english guide!

[@oscarchan5684]

Thank you for such a clear explanation!!!👍🙏

[@AbhishekJaiswal24]

Thanks neetcode !!!

[@shaon6996]

Great explanation!

[@vishalanthony1529]

This is one of those rare videos of Neetcode where I could not really understand what he is trying to convey

[@shrimpo6416]

great explanation!!!

[@jameszhang9422]

Thank you for the great video!

[@nemesis_rc]

Your channel is really underrated man 😢

[@TokyoXtreme]

Truth.

[@juliewiner5287]

but how do you know the difference will be contigous with the current sum ?

[@nguessaneric9830]

Because you substract a prefix, you don't select element in your window

[@ygwg6145]

It appears easier to use hash table cum->deque[index]. It did pass all the tests.

[@chilly2171]

9:04, shouldn't it be 3 instead of -3? since you need to add 3 to 0 to form K.

[@nemesis_rc]

Explained well 👍

[@aadil4236]

10:02 Why didn't we add 2 as a prefix there? Was that a mistake on your part? I'm confused.

[@andrewsowell3088]

Yep, should have been added

[@asdfasyakitori8514]

Great video

[@LogicLoop8]

Why doesn't sliding window approach fails here: - Assumption: adding an element to increase and removing an element to decrease any sum is valid only if all elements are non-negative. - removing negative elements increases total sum and vice versa, hence breaks tests containing negative number - not suitable approach for this problem

[@ryanc7840]

Thank U. A huge kudos!

[@NeetCode]

Glad it helped!

[@gorilla4870]

Simple problem, but can be quite confusing to explain. You explained it very well Neet. Thanks a lot!

[@saul_no]

I was actually asked this question for a Facebook internship position in 2019.

[@dmwhite4787]

Did you find the optimal solution in an interview? Had you seen this problem before? If no, did you pass that round?

[@immortalized_onion]

This reminds me of the two sum solution.

[@4r4zzz]

Thanks a lot

[@mike0998unstopable]

Wayyyy less confusing than the explanation on leetcode