pritishn's blog

By pritishn, 7 weeks ago, In English

demoralizer and I were discussing our 2+ years old code and I thought it'd be a good motivation for newbies and pupil who write cancer currently.

Please contibute by sharing code(in spoiler tag) or links to your of your big brain moments from back when you were low rated.

My biggest big brain moment was when I assumed inbuilt std::sort must be slower, so copied merge sort from gfg. https://www.codechef.com/viewsolution/23270721

 
 
 
 
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7 weeks ago, # |
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This is where this discussion began:

When I was too dumb to use loops
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7 weeks ago, # |
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assumed inbuilt std::sort must be slower atleast you had knowledge of stl,I used to write gcd ,sort functions on my own and instead of pair I was using 2d array.

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7 weeks ago, # |
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newbies and pupil who write cancer currently.

Don't underestimate me, I still write cancer solutions to simple problems. (I remember spamming maximum bipartite matching on a div2 B in-contest.)

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    7 weeks ago, # ^ |
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    I once used Backtracking + CircularLinkedList + Sets and Randomization to solve a div2 A, felt empty despite the AC, but at that moment, not solving A would've been harder to digest. submission

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7 weeks ago, # |
Rev. 2   Vote: I like it +66 Vote: I do not like it

As someone already pointed out, all of us write cancer solutions from time to time.

For example, this problem from the ICPC WF Invitational mirror had a trivial $$$O(n^2)$$$ solution, but I didn't read the constraints and used lex and yacc to generate this monstrosity of a solution that works in $$$O(n)$$$ instead.

Edit: since someone said the submission isn't visible, here you go.

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7 weeks ago, # |
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I remember one time I solved a rather tough problem for my level but ended up not implementing it because I thought time complexity for nCr is O(n) and not O(r).

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I thought writing a $$$O(n^2)$$$ in cpp instead of java would get me an Accepted solution.

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    7 weeks ago, # ^ |
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    In some special cases you can actually do it as there are some optimization tricks available on C++

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      7 weeks ago, # ^ |
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      Not quite sure why am I wrong here. It is not all the cases but rather than special cases.

      Such as:

      • The case when $$$O(n^2)$$$ is enough and intended solution yet your constant factor is bigger than the author.
      • There is a difference in some functionalities that you can use to improve the speed
      • Not kidding but some C++ template functions is actually very good compared to some java functions that you can use for problem like flow, matching, fft, or geometric, polygon stuffs for one that dont have
      • Friendly Cache Implementation
      • Constant Optimization
      • Pragma Vectorization
      • Bit-hack Tricks
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7 weeks ago, # |
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I thought that binary search is so slow in special cases so I use random middle pivot to boost it in those cases :)

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7 weeks ago, # |
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I thought inserting #pragmas in my code will magically make my code fast, so I should not bother about time complexities anymore!

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7 weeks ago, # |
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I thought maps are magical. Just insert elements into a map, and voila, you have sorted the input in O(n). I remember telling this to my friend and then proceeding to have a clown moment for the rest of the day.

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    7 weeks ago, # ^ |
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    Well, if you consider frequency array as map...

    int mpp[maxN]={0};
    for(int i=0;i<n;i++)
    {mpp[a[i]]++;} //O(n) 
    for(int i=0;i<maxN;i++)
    {while(mpp[i]--)cout<<i<<",";}//O(maxN)
    

    O(n+maxN) is O(n)
    Yet another big brain moment xD

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      7 weeks ago, # ^ |
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      I guess this is called bucket sort if I am not wrong . I once used it in a leetcode problem !!

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7 weeks ago, # |
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I thought most of the $$$O(n^2)$$$ solutions would pass if I use correct pragma :(

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7 weeks ago, # |
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I thought printing sample test cases output will give me accepted

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I used to think set operators like |(Union),&(Intersection) in python has O(1) time complexity.

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I used to learn Z-function by heart without understanding how it works at all before I attended offline competitions. I was afraid of tasks about strings and thought that somehow z-function might save me. It did not work out even once)

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string a, b;

1. a = a + b;
2. a += b;

I used to think both 1 and 2 were equally efficient.

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    7 weeks ago, # ^ |
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    I still think they are equally of same time complexity :( Aren't they?

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      7 weeks ago, # ^ |
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      Let $$$n$$$ be the size of $$$a$$$ and $$$m$$$ the size of $$$b$$$, then the first one is $$$O(n + m)$$$ and the other is $$$O(m)$$$.

      This is because it returns a new string by doing a + b. a += b on the other hand, just appends b to a, it's like push back in vector. As a side note, push back also works there: a.push_back(b).

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        7 weeks ago, # ^ |
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        It's not strictly $$$O(m)$$$, but it's $$$O(m)$$$ on average and $$$O(n+m)$$$ at max. That's also important to know, though I don't have a sample of code, where it matters.

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        7 weeks ago, # ^ |
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        Thanks for clarification

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Younger me refuses to acknowledge the existence of arrays...
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Wrote an solution with exponential complexity where $$$n = 2e5$$$, got TLE, searched Google: How to speed up program in C++ then added ios_base::sync_with_stdio(false); cin.tie(NULL);, spent the rest of the contest wondering why I still got TLE...

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Will let you know once I become high rated :).

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When in my first contest on codeforces (div3 round) I got +500, I thought I will become red after few more contests.

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Anyone else once thought that vector.insert works in $$$O(1)$$$?

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me thinking that $$$O(n^2)$$$ is faster than $$$O(n log n)$$$

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Those cases in which I was getting TLE or WA on gfg, I just copy those cases, and put if condition for those and print the result in O(1)

Spoiler
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7 weeks ago, # |
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I had some weird obsession with macros.. warning: disturbing content below

look
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7 weeks ago, # |
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I used to think that std::lower_bound in a set was $$$\mathcal{O}(\log N)$$$ and surprisingly solved many problems, until the day when I TLE'd

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    7 weeks ago, # ^ |
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    Didn't you mean std::lower_bound on set?

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    7 weeks ago, # ^ |
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    Umm.. Noob here ! Isn't lower_bound operation on a set or sorted vector O(log n) ??

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      7 weeks ago, # ^ |
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      For sorted vectors it's logarithmic but for sets it's linear
      std::lower_bound on set

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        7 weeks ago, # ^ |
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        Thanks for the reply!!

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        7 weeks ago, # ^ |
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        Are you sure?, I still think that it's logn and used it many times and didn't got TLE.

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          7 weeks ago, # ^ |
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          As mentioned here, std::set doesn't has random access iterators, so the time complexity is linear.

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          2 weeks ago, # ^ |
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          If you have a set S, then "S.lower_bound(number)" is O(logN), but "lower_bound(S.begin(), S.end(), number)" is O(N). One is a member function, the other is a generic function in the "algorithm" header. Make sure to not mix these up!

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    7 weeks ago, # ^ |
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    Lesson: use std::set::lower_bound instead of std::lower_bound on set

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Another edition of "I still write cancer":

Switch case is something that I will not use in 100 years. If else it is. So at my latest test at the university, we had a trivial problem with dates (day,months,years) and some operations with them. So my code has like a million of:

if (m == 1 || m == 3 || m == 5 || m == 7 || ) ...
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Not high rated, but in my first contest which was a google kickstart, I didn't knew that godly things like std::sort exist, and as I just completed the MIT course on algorithms (because for some reason I thought I'd clear IOI by watching it), I know heap sort is the fastest thing I know so I copied the heap sort code from gfg (that too in java cuz aman dhotarchor said java is better) and ended up in getting an RTE which I spent whole hour figuring out.

Second one: my first contest on CF was by coincidence an April Fools round and I ended up solving three problems, so I thought I am cyan level from the start

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Binary search
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learning not basic data structures while being green

also tried reading about splay tree when i was 1300 to flex on my friends with it

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7 weeks ago, # |
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When I heard people talking about "node", I thought it was some graph shits. Turned out to be nodeJS.

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7 weeks ago, # |
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I used to think rating was directly proportional to how many macros you used.

Viewer discussion is advised
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7 weeks ago, # |
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Wrote an exponential solution to this problem and it passed pretests and FSTed. I thought I solved a Div2 C for the first time :(

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I used to believe std::lower_bound and std::upper_bound is all that binary search algorithm is.

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i thought $$$\frac{10^6}{2} = 10^3$$$ in a contest and proceeds to spend whole contest wondering why my code got TLE.

and this has happened more than once :)

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7 weeks ago, # |
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I thought the compiler on cf wont be able to tell what the result of program was. So I quoted with the output

Spoiler
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7 weeks ago, # |
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And then also you got WA :)

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    7 weeks ago, # ^ |
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    Me doing binary search as: checking mid element and if mid<element : then doing linear serach from 1 to mid else linear search from mid to n

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To get the minimum integer of an array I used *min(vec.begin(), vec.end()). To test this I took a "random" array in which the first element was the minimum of the array :). So it took me a long time to figure out what was wrong.

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I used to think that stl size() function has complexity of O(n) and always stored the size in a variable before writing a loop.

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I once thought I had invented an O(n) sorting algorithm. Didn't know time complexity of map back then.

Spoiler
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    2 weeks ago, # ^ |
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    -

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      2 weeks ago, # ^ |
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      You can't maintain sorted order in unordered_map :p

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        And that's what happens when you comment before coffee! Yeah no way that works

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56589098, I solve this during the contest when I was newbie, a Few days later I got that this was a "prefix sum" approach.

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A fairly long time ago I had no idea functions exist and wrote everything in the main()

Without an understanding how to do functions, when I encountered any problem with some graphs, I would be trying to make graph traversal algorithms using for loops, gotos(yeah...), and bools, all in main(), and it was WILD.

I remember how I would have 5 nested for loops and a million ifs inside and would be thinking "why does it have to be so complicated". Debugging was a total nightmare, and it was more or less like "if i don't fix this within an hour i give up". Unfortunately i cant get any code from that time cause it was on an old laptop that doesnt work anymore lol.

You can imagine how it felt to find out about recursions and dfs lmao.

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I once had an $$$O(n)$$$ solution to a problem. Complexity was ok but the problem had $$$1e5$$$ testcases(so solution must be calculated in a single $$$O(n)$$$ pre-processing). Got a TLE on pretests so I made a global map which stores the values after printing them in each testcase. Passed pretests and FSTed.

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7 weeks ago, # |
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More than 6.5 years ago. Must've been doing competitive programming for 7-8 months or so, I didn't know about digit dp. Was trying to solve https://projecteuler.net/problem=164, so I ended up writing code whose output was the code that solves this problem. https://ideone.com/XgqZDh

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3 weeks ago, # |
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What is the best resource to learn CP topic wise

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3 weeks ago, # |
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I used Pascal

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Recently I invented centroid decomposition on array

(I forgot that it's called segment tree)

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Back when I didn’t know what the Sieve of Eratosthenes was, I used to make an array with all the prime numbers within a range for any solution that required them. Ex. 104820322

Needless to say, the speed at which I came up with this type of solution was awfully slow.

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I used t think that we need to learn whole C++ along with all the Data Structures and Algorithms in the world before starting Competitive Programming, as the word Competitive seemed too scary. I used to think that only the best coders need to compete in order to deicide the best of the best, like in the Olympics.

Thanks to my college seniors for kicking my ass right from the beginning, I didn't end up making that mistake.

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never trusted function swap(a, b); I always wrote:

my code:
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3 weeks ago, # |
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When I started to code, I don't know why it was hard for me to digest that a number can have a max of 1 prime factor above sqrt(n) and writing sieve with proper understanding at first time was also a very complex task for me.

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Used to generate short permutations without using array.

code
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I thought printing sample cases brings me above those who do not attempt the questions and gives me + rating.

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I used to be obsessed with doing the XOR trick to swap two integers w/out a dummy variable. I thought it was a galaxy brain thing to do.

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Never used local variables, i always used global variable and my code looked something like this:

my code:
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Once doing a problem during some contest I was too amateur to understand what sum of k over test cases meant . So had an idea to solve the problem using two nested for loops one for N and another for K . Thought the complexity was O(N*K) and it would Tle , so didnt proceeded with the solution only to find out editorial had the same solution and spent like two weeks wondering why it would not Tle

Later on understood complexity was O(N+K) because it was written sum of K over all test cases was less than 1e5

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I used to assign whatever phrase came to mind at the time to strings whenever I needed a way to set them to null.

I also thought array indices started from 1.

Enjoy: 26783948

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I am still low rated

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2 weeks ago, # |
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thought that problems were stupid and C++ was too slow because my O(n^2) solution gives TLE.

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learned dinic's flow algo while struggling to get out of green!