### maroonrk's blog

By maroonrk, history, 3 weeks ago,

We will hold AtCoder Regular Contest 145.

The point values will be 400-500-600-700-800-1200.

We are looking forward to your participation!

• +155

 » 3 weeks ago, # |   +32 Looking forward to ARC !!
•  » » 3 weeks ago, # ^ |   +1 me too
 » 3 weeks ago, # |   +151 As the writer, good luck and enjoy yourself! :D
•  » » 3 weeks ago, # ^ |   +1 As a contestant, I want to be blue :)
•  » » 3 weeks ago, # ^ |   +9 Can I do up-hack to my AC submission? This submission can be hacked by n=4 AAAA
 » 3 weeks ago, # |   +1 What's the approximate rating of ARC's problems B & C according to CodeForces rating distribution?
 » 3 weeks ago, # |   +19 math and counting round
 » 3 weeks ago, # |   +6 I really suck at counting, but today I couldn't figure out what the 16 different permutations were in C, the most I found was 12 :PCan anyone list those out please?
•  » » 3 weeks ago, # ^ |   +22 1 2 3 4 2 1 3 4 1 2 4 3 2 1 4 3 3 4 1 2 4 3 1 2 3 4 2 1 4 3 2 1 1 3 2 4 2 3 1 4 1 4 2 3 2 4 1 3 3 1 4 2 4 1 3 2 4 2 3 1 3 2 4 1 
•  » » 3 weeks ago, # ^ |   +1 Code that prints int n = 2; vector p(2 * n); vector pr(2 * n + 1); for (int i = 1; i <= 2 * n; i += 2) { pr[i] = i + 1; pr[i + 1] = i; } iota(p.begin(), p.end(), 1); int ans = 0; do { vector place(2 * n + 1, -1); vector open(2 * n); for (int i = 0; i < 2 * n; ++i) { place[p[i]] = i + 1; if (place[pr[p[i]]] == -1) { open[i] = true; } } bool now = true; int last = -1; for (int i = 0; i < 2 * n; ++i) { if (open[i]) { if (last > place[pr[p[i]]]) { now = false; break; } last = place[pr[p[i]]]; } } if (now) print(p); } while (next_permutation(p.begin(), p.end())); 
 » 3 weeks ago, # |   +1 Could someone point out what case am I missing in my submission for B?
•  » » 3 weeks ago, # ^ |   +17 I believe you miss the cases where $N < A$.
•  » » » 3 weeks ago, # ^ |   +4 Thanks for pointing that out.
 » 3 weeks ago, # |   0 My first AtCoder contest. The problems were good, it was interesting for me. Thanks !
 » 3 weeks ago, # |   +65 D is just cantor set...
 » 3 weeks ago, # |   +10 As a contestant, I was not a big fan of A,B,D. Those problems' ideas are not hard to get, but have very annoying corner cases...
 » 3 weeks ago, # |   0 Problem D is magic, but I want to know, why the set ${1,2,4,7,11,...}$, which the differential sequence is ${1,2,3,4,...}$ is not the optimal one.
•  » » 3 weeks ago, # ^ |   +19 The set $1,2,4,7,11,\dots$ has an $\mathrm{O}(n^2)$ value on the $n$-th element, while the optimal set has $\mathrm{O}(n^{\log_2 3}) \approx \mathrm{O}(n^{1.585})$ value on the $n$-th one ,which is enough to meet the constraint of $[-10^7,10^7]$ .
•  » » » 3 weeks ago, # ^ |   0 U R right.I tried that,and the number can be as large as 4e7
•  » » 2 weeks ago, # ^ |   +30 That doesn't even work, $1+7=2\cdot 4$
 » 3 weeks ago, # |   +16 Sequence of pC can be found here.
 » 3 weeks ago, # |   +24 My friend told me the problem C is in the OEIS, what do you think of it?
 » 3 weeks ago, # |   0 void jaiishriRam(){ ll n; cin>>n; string s; cin>>s; if(n==2) { if(s=="BA" || s=="AB") { cout<<"NO"<
•  » » 3 weeks ago, # ^ |   +8 You have to print "Yes", not "YES". Same for "No" and "NO"
•  » » » 3 weeks ago, # ^ |   0 Ohh tnx!
 » 3 weeks ago, # |   +16 Can anyone plz tell me whats wrong here in my submission for problem B
•  » » 3 weeks ago, # ^ | ← Rev. 2 →   +1 Consider the test case :- 6 5 4 , Your O/P — 4 Correct O/P — 2 for value (5 and 6 only Alice will win )
•  » » » 3 weeks ago, # ^ |   0 Corrected that mistake but still getting WA on 3 test cases ;-; Here is my submission
•  » » » » 3 weeks ago, # ^ | ← Rev. 2 →   +1 I'm not sure if binary search is a good idea here. You can try 30 10 3 correct ans: 7 your code gives: 9
•  » » » » 3 weeks ago, # ^ |   +1 Consider this test case :- 8 4 3 Your O/P — 6 Correct O/P — 4 Aice wins game (4,5,6 and 8) Error is in line 58 loop that you are multiplying . They will depend on remainder of (n%a).
•  » » » » » 2 weeks ago, # ^ |   0 Thank you guys so much, my code got AC , there was implementation mistake at the end . In case anyone wants to see binary search solution — Here
•  » » 3 weeks ago, # ^ | ← Rev. 2 →   0 your code doesn't handle the cases for $n$ in the range $a \le n < a+b$
 » 3 weeks ago, # |   +8 Can anyone please tell me whats wrong here in my submission for problem D?
•  » » 3 weeks ago, # ^ |   +16 I don't know how your code works, but it fails at the input 4 2
•  » » » 3 weeks ago, # ^ |   +8 thx.
•  » » » 3 weeks ago, # ^ |   +8 It works in such way: $\pi(n)\sim \frac n{\ln n}$So the dfs is expected to stop in very low depth.p.s. What about this one? It could pass 4 2.
•  » » » » 3 weeks ago, # ^ |   0 It fails at 10000 0.The output has these three integers: -9999991 -9999937 -9999883 
•  » » » » » 3 weeks ago, # ^ |   +8 Sorry, but it got 9999991 -9999991 9999973 -9999973 9999971 ... here.Maybe there's some Undefined Behavior in my code?
•  » » » » » » 3 weeks ago, # ^ |   +8 It is not a UB, because the output is a set, and these three numbers are in it, which is against the constraint. Actually, they are not the first three numbers of the output on my computer as well.
•  » » » » 3 weeks ago, # ^ |   +27 Are you assuming primes don't have arithmetic sequences of length 3? Because that is not true, for example: $101, 107, 113$.
•  » » » » » 3 weeks ago, # ^ |   +8 Oh, that's it.Sincerely thanks for your help!
 » 3 weeks ago, # |   +40 C could be solved by writing a brute force and searching the sequence on OEIS. Maybe those problems should be avoided by for example having problems parametrized by some more values, or making sure it isn't easy to find on the internet. It makes the contest experience less fun, if the answer is easily found on the internet. Problems A and B were fine. Problem D also suffered from this issue.
 » 3 weeks ago, # |   +18 C is actually A152029(OEIS).
•  » » 3 weeks ago, # ^ |   0 Whoa, i didn't brute force on the actual answer tho, I bruteforced on a subproblem of it, The Catalan Numbers
•  » » » 3 weeks ago, # ^ |   -11 Catalan numbers are well known.
 » 3 weeks ago, # | ← Rev. 3 →   0 Why does this code give WA for problem A? lng dp[3][SZ],n;string s; lng solve(lng i,lng x) { if(i==n-i){return 1;} if(i>n-i){return (x==0);} if(dp[x][i]!=-1){return dp[x][i];} lng j=(n-1)-i; char a=s[i],b=s[j]; if(x==1){a='B';} if(x==2){b='A';} if(a==b) { lng z=solve(i+1,0); if(a=='A'){z=max(solve(i+1,1),z);} else{z=max(solve(i+1,2),z);} return dp[x][i]=z; } if(j-i==1){return 0;} if(a=='A'){return 0;} return dp[x][i]=max(solve(i+1,1),solve(i+1,2)); } int main() { ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0); cin >> n >> s; for(lng i=0;i<3;i++){for(lng j=0;j
 » 3 weeks ago, # |   0 This was my first contest on AtCoder, Can someone tell me, when do the editorials be out ?
•  » » 3 weeks ago, # ^ |   +3
•  » » » 3 weeks ago, # ^ |   0 Thank you so much sir!
 » 3 weeks ago, # |   +8 nok0 My submission for A get AC but can be hacked with n=4 AAAA My submission
 » 3 weeks ago, # | ← Rev. 2 →   +3 In problem A:Why is "AB" a valid palindrome or can be changed to a palindrome according to editorial and tests?
•  » » 3 weeks ago, # ^ |   0 AB doesn't satisfy the first condition (s[0] == "B" or s[-1] == "A") in the editorial.
 » 3 weeks ago, # |   +8 for D you can just use brute force to get a set meet the last restriction,and change the largest/smallest number,then add x to all numbers to get sum=Mbase 3 solution is beautiful but not necessary
•  » » 3 weeks ago, # ^ |   0 Can you share your submission?
 » 3 weeks ago, # |   0 My solution to B in the contest seems to be wrong.When $nb$, Alice can win when $(n\bmod a)  » 3 weeks ago, # | +117 I have different approaches for problem D and E:D:Ingore the limit of sum for now. Let$f(n)$be the set of integers, one can find that$ f(1)=\{0\} \\ f(2n)= f(n) \cup (f(n)+2\max f(n)+ {\color{red}1}) $is good. Now consider the limit of sum. One can find that if we change the$\color{red}1$into$2$, and add$1$to some greatest elements in$f(n)$, it is still good.E:We can change$A_j$to$A_i\oplus A_j$($i
•  » » 2 weeks ago, # ^ |   +100 Your code on E submitted during contest can be hacked using the data generated by the code below. #include #define rep(i,a,n) for (int i=(a);i<=(n);i++) int main() { int n = 1000; printf("%d\n", n); rep(i, 0, 59) printf("%lld ", 1ll << i); rep(i, 1, n - 60) printf("%lld ", (1ll << 60) - 1); printf("\n"); rep(i, 0, 59) printf("%lld ", 1ll << i); rep(i, 1, n - 60) printf("%lld ", (1ll << 60) - 1 - (i & 1)); return 0; } 
•  » » » 2 weeks ago, # ^ |   +6 Thanks for pointing this out! I tried to add some randomization and it worked pretty good.
 » 3 weeks ago, # | ← Rev. 2 →   +39 My randomized solution for E:If the answer is Yes, we first randomly manipulate the sequence for a small number of times, and then operate on index $n$ more than $n$ times repeatedly. Then we can (roughly) assume that $a_n \oplus b_n$ is a linear combination of $a_{n-r+1 \cdots a_{n-1}}$, where $t$ is about $60$.We can then solve $a_n \oplus b_n = \bigoplus_{j \in S} a_j$ with Gaussian elimination. If $a_{n-1} \not \in S$, operate on $n$ so that $a_{n-1} \in S$.Then we operate on the second minimum element $t$ in $S$. Then $\min S$ is removed from $S$ and all indices between $\min S$ and $t$ are added. Repeat this process until $S = {a_{n-1}}$ and operate on $n$. Do this for $n - 1 \dots 2$ and we are done. This solution uses ~60000 operations in randomly generated inputs.implementationUPD: More than 1000 random operations at first are needed to guarantee randomness.
 » 3 weeks ago, # |   0 Can somebody explain here problem D, how exactly are we constructing the required set after we have figured out the base 3 condition where each digit should be 0/1.
 » 3 weeks ago, # |   +10 F is some wild generalization of IMO 1995 P6. I like it
 » 3 weeks ago, # |   +5 Such a magical problem D! Base-3 representation is such a trick that could perfectly beat the rule of $2y \neq x + z$.Next, by first keeping the rightmost digit of all n integers as zero and then change some of them to 1, so that $|m - sum| \% n = 0$. At the same time, all n integers are still distinct and satisfy the "good set" rule.Finally, add $(m - sum) / n$ to each integer so that $sum = m$, and at the same time, adding the same integer will not lead to any $2y = x + z$, although they may not belong to the "good set" anymore.Thanks to the problem writer for coming up with such a clever and unbelievable construction problem!
 » 3 weeks ago, # | ← Rev. 2 →   0 Was only able to solve A, although I misread the problem at first and somehow thought that I could use the operation only when adjacent characters are equal, lead to 3 WAs but we move....
 » 3 weeks ago, # |   0 In terms of codeforces ratings, what would be the difficulty rating for problem A?
 » 3 weeks ago, # |   0 can anyone explain logic of c plz?
 » 2 weeks ago, # |   0 This round is good, arc should contain magical counting and constructing problems like this.
 » 2 weeks ago, # | ← Rev. 4 →   0 A different way to think for D.We can construct as follow (P is a set,initial $P={1},len=1$): $P\to P\cup (P+(2len-1)),len\to 3len-1$$P+a$ means add $a$ to all elements in $P$.And it can prove that $y−x\not=z−y$ for every triple $x,y,z (x •  » » 2 weeks ago, # ^ | 0 Could you please talk more about why your construction will not lead to 2y=x+z? And, how to construct these n+2 elements, and how to determine which two to delete?Thank you so much for providing such a clever idea. •  » » » 2 weeks ago, # ^ | ← Rev. 3 → 0 Let$P'=P+(2len-1),\forall x\in P,y\in P,z\in P',$we can found that$|x-y|
 » 11 days ago, # |   +1 Sorry for asking it late,Can any one please the explain this line of editorial : Here, if the elements in P are scanned from left to right, there must always be more left elements than right elements. The number of such arrangements corresponds to that of parenthesis sequences.What I understood is that, lets say we have (1,2),(3,4),(5,6),(7,8) as corresponding (ai and bi).Permutation : (1,2,4,3,5,6,7,8) have more right elements(2) than left elements(1) in first 3 elements (1,2,4) but still it can be divided it into two sequences A(1,3,5,7) and B(2,4,6,8), but in editorial it is told that it should not be possible ( as per I understood ) . I know I have understood it wrong. Anyone who did this problem or got the editorial please explain the line in blueEditorial link : Link