m3tr0's blog

By m3tr0, 3 weeks ago, translation, In English

2036A - Quintomania

Idea: m3tr0

Tutorial
Solution (myav)

2036B - Startup

Idea: Seny

Tutorial
Solution (Seny)

2036C - Anya and 1100

Idea: m3tr0

Hint
Tutorial
Solution (m3tr0)

2036D - I Love 1543

Idea: eugenechka.boyko.2_0-0

Tutorial
Solution (m3tr0)

2036E - Reverse the Rivers

Idea: m3tr0

Hint
Tutorial
Solution (m3tr0)

2036F - XORificator 3000

Idea: eugenechka.boyko.2_0-0

Hint 1
Hint 2
Tutorial
Solution (eugenechka.boyko.2_0-0)

2036G - Library of Magic

Idea: m3tr0

Hint 1
Hint 2
Tutorial
Solution (m3tr0)
  • Vote: I like it
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  • Vote: I do not like it

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3 weeks ago, # |
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make better pretests next time.

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3 weeks ago, # |
Rev. 2   Vote: I like it 0 Vote: I do not like it

can anyone please explain what is logically wrong in this code: https://codeforces.me/contest/2036/submission/289551678 (it is failing on test case 35). thanks in advance!

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3 weeks ago, # |
  Vote: I like it +11 Vote: I do not like it

Problem F can also be solved using digit dp.

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    3 weeks ago, # ^ |
      Vote: I like it +4 Vote: I do not like it

    i just read your lines that was really nice in the ending

    Mar Gaye Ehsaas Meray, Chubay Wo Ilfaaz Teray, Tumne Dekha Tak Na Jaty Way,

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    2 weeks ago, # ^ |
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    I read a little bit of that code and it seems the code cant really solve for a number NOT a power of 2 (the constant time solution doesnt either). If I understand correctly you are checking if the first i bits of the number are different or not and xorring the remaining numbers. Can you extend your solution to range xor with a similar constraint but % x where x can be anything?

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      2 weeks ago, # ^ |
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      Yes You are exactly correct, this code only handles when x is a power of 2. I had thought about the case when x is not a power of 2 and I could solve it for small values of x only($$$<=1000$$$) by having an additional state for the current remainder of the prefix.

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        2 weeks ago, # ^ |
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        I couldn't find a better solution either, the best I could do was the same as you, I guess I should attach my code in case anyone in the future wants to know.

        Code:
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    2 weeks ago, # ^ |
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    can you explain your solution, especially how you are ensuring that the remainder is not k ?

    Thanks in Advance!

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      2 weeks ago, # ^ |
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      A number mod power of two, $$$2^i$$$ is just the last $$$i$$$ bits in it's binary representation.

      We can use digit dp in the following way: Suppose you want to build an n digit binary number. Pick a digit (0/1 in this case) and place it at the leftmost space, you are left with n — 1 digits, now you need to count how many numbers are possible to be built in total after placing those n — 1 digits. To do this simply recursively start to add digits and keep track of whether the number you have built so far is a valid number or not. When you reach the end of the number, if the build is valid you return 1, otherwise 0. You can very efficiently count the possible valid numbers you can place in those n — 1 spots.

      Now if the count is odd then you know that your current digit (which you picked first) will appear in the net xor sum (If the picked digit was 1 of course, if it was 0 then it won't appear anyway), hence you can store this net xor sum by the following line:

      $$$xsum_i$$$ ^= (1 << (n — i — 1)) ^ $$$xsum_{i + 1}$$$ for both the digits 0/1 which you put in ith spot. This represents the transition.

      To store both the count of how many times the build starting from ith place to the end appears and xor of all such numbers can be stored separately in two different arrays or you can just use a pair or array<int, 2> like I did here: 290112812

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3 weeks ago, # |
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can anyone please tell me whats wrong with this solution 289604455

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3 weeks ago, # |
  Vote: I like it +27 Vote: I do not like it

BitForces

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3 weeks ago, # |
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If you don't know how to make testcases, don't make contests only for fun . L contest

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3 weeks ago, # |
  Vote: I like it -16 Vote: I do not like it

289481978 why is this wrong , this is O(klogk)

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    3 weeks ago, # ^ |
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      3 weeks ago, # ^ |
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      but why did it give tle

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        3 weeks ago, # ^ |
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        In short, carefully constructed inputs will lead to hash collisions when inserting into/updating a dictionary in Python, and that results in a significantly worse time complexity and thus TLE (each individual insert/update can be O(n)). The link outlines how to avoid this issue. For this problem, you can also use a list/array.

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3 weeks ago, # |
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289592707 why is this wrong , can someone tell ?

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    2 weeks ago, # ^ |
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    because you break the loop while reading, it will cause your next query reading the wrong input.

    Instead of if l>=r then break, use if l<r then solve()

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3 weeks ago, # |
  Vote: I like it -10 Vote: I do not like it

Alternate Solution for Problem G

Let $$$\text{XOR}(1, n)$$$ denote the XOR of all elements from the first to the $$$n$$$-th element in the array.

Case 1: If $$$\text{XOR}(1, n) = 0$$$, the numbers are structured to cancel out in pairs, leaving us with three values: $$$x$$$, $$$y$$$, and $$$x \oplus y$$$. We then query $$$\text{XOR}(1, h)$$$ for each $$$h$$$ starting from $$$h = 1$$$ until we find the smallest $$$h$$$ such that $$$\text{XOR}(1, h) \neq 0$$$. At this point, $$$\text{XOR}(1, h)$$$ isolates the first missing element $$$x$$$ because all other numbers in this subrange occur in pairs and cancel out.

Case 2: If $$$\text{XOR}(1, n) \neq 0$$$, we aim to locate the smallest subarray where $$$\text{XOR}(1, \text{mid}) = 0$$$. We perform a binary search to identify this position, isolating the unpaired number in that range. This number, $$$x$$$, is then equal to $$$\text{XOR}(1, \text{low})$$$ where low marks the boundary.

After identifying $$$x$$$, we search for $$$y$$$, the second unpaired element, starting in the range $$$[x+1, n]$$$:

We perform XOR queries from $$$a + 1$$$ to $$$\text{mid}$$$ until encountering a position where $$$\text{XOR}(x + 1, \text{mid}) \neq 0$$$. This value $$$y$$$ is isolated by $$$y = \text{XOR}(a+1, \text{low})$$$.

Finally, the third number $$$z$$$ can be computed as: $$$z = \text{total_xor} \oplus x \oplus y$$$

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    2 weeks ago, # ^ |
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    Nice idea ! can u tell me why u used

    (in case where xor of 1 to n is 0 , I had same idea for everything else but couldnt figure out how to find first XD)

    while (query(1, temp) == 0)
            {
                temp = 2 * temp + 1;
            }
    

    instead of

    while (query(1, temp) == 0)
            {
                temp = 2 * temp;
            }
    

    The 2nd one is giving TLE , why it is guaranteed that one of them has 1st bit set ?

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      2 weeks ago, # ^ |
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      when you are using former, it's guaranteed that the range has odd number of elements meaning (the pairs+the alone element), but if it has even number of elements in range the it would have (even number of pairs + 2 elements once, and would be the starting of another pair/the other removed element), so we won't really be able to find the solution via 2nd approach.

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3 weeks ago, # |
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please try to improve your pretests next time

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3 weeks ago, # |
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Could someone explain what went wrong for me in this submission for D?: https://codeforces.me/contest/2036/submission/289572926

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    3 weeks ago, # ^ |
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    The loop for going over all spirals should be till min(m, n)/2 not n/2. I changed this part of your code and submitted it. Got AC.

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3 weeks ago, # |
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I think F can be solved in O(1) time instead of O(log r) since bitwise XOR runs in constant time.

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3 weeks ago, # |
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F is very nice, solved it using some bit tricks, but was thinking about some digit dp solution does it exits ? like dp(n, tight, flag) building n digit number with tight telling us that it is smaller than (say r) and flag telling us about if to take the new string number formed?

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    3 weeks ago, # ^ |
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    Code something like this
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3 weeks ago, # |
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So my code gets accepted when I use a 2D char array but not when I use vector<string>

(Fails on testcase 2)

Any idea what might be causing this?

Thanks!

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2 weeks ago, # |
Rev. 3   Vote: I like it 0 Vote: I do not like it

UPD: Figured. I was breaking out of the loop at the end, which was skipping inputs.

For Problem E, the code works fine for test 9 (and gives correct output) on my PC as well as custom invocation (attached) but it gives RTE (on test 9) when I submit it. I am not able to find any undefined behaviour in my code. Can someone help me where the potential RTE could be in my code?

Submission

image

image

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2 weeks ago, # |
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Can someone point out the logical errors in my code? Thanks. It's wrong on test 4, case 51.

void solve(){
    long long n; cin >> n;
    auto query = [&](long long l, long long r){
        cout << "xor " <<  l << ' ' << r << endl;
        long long res; cin >> res;
        return res;
    };
    auto solve = [&](long long lo, long long hi){
        if(lo > hi) return -1LL;
        while(lo < hi){
            auto m = lo + hi >> 1;
            if(query(lo, m)){
                hi = m;
            }
            else{
                lo = m + 1;
            }
        }
        return lo;
    };
    long long tot = query(1, n);
    long long A = -1, B = -1, C = -1;
    // find 2 ranges of A and B 
    long long p = 1;
    for(long long (*x) : {&A, &B}){
        long long next_p, l, r, res;
        while(p <= n){
            next_p = p * 2;
            l = p;
            r = min(next_p - 1, n);
            res = query(l, r);
            p = next_p;
            if(res) break;
        }
        long long y = solve(l, r);
        *x = y;
        if(y != res){
            if(B == -1){
                B = solve(y + 1, r);
                if(B == -1){
                    solve(l, y - 1);
                }
                if((B ^ A) != res){
                    C = res ^ A ^ B;
                    
                }
                break;
            }
            C = res ^ y;
            break;
        }
    }
    if(C == -1){
        C = tot ^ A ^ B;
    }
    // if((A ^ B ^ C) != tot){
    //     cout << "WR " debug(A) debug(B) debug(C) debug(tot) << endl;
    // }
    cout << "ans " << A << ' ' << B << ' ' << C << endl;
}

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2 weeks ago, # |
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Seeing that many hacks in div3, especially in ABCDE problems is hilarious. I hope next time pretests will be much better. Great contest anyway.

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2 weeks ago, # |
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can anyone help me... i really dont understand why should this be giving a wrong answer https://codeforces.me/contest/2036/submission/290070503

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2 weeks ago, # |
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A simpler solution to the problem D. Hope this helps!!!

---CODE---


[problem:2036D]#include <bits/stdc++.h> #include<cmath> using namespace std; string s[1000]; string curr; void fun(){ int n,m; cin>>n>>m; for(int i = 0 ; i<n ;i++){ cin>>s[i]; } int top = 0; int bottom = n-1; int left = 0; int right = m-1; int count = 0; while(top<=bottom && left<=right){ curr.clear(); for(int i = left ; i<=right ; i++){ curr.push_back(s[top][i]); } top++; for(int i = top ; i<=bottom ; i++){ curr.push_back(s[i][right]); } right--; for(int j = right ; j>=left ; j--){ curr.push_back(s[bottom][j]); } bottom--; for(int j = bottom ; j>=top ; j--){ curr.push_back(s[j][left]); } left++; curr += curr.substr(0,3); for(int i = 0 ; i<=curr.size()-4 ; i++){ if(curr[i] == '1' && curr[i+1] == '5' && curr[i+2] == '4' && curr[i+3] == '3'){ count++; i = i+3; } } } cout<<count<<endl; } int main() { std::ios::sync_with_stdio(0); std::cin.tie(0); int t; cin>>t; while(t--){ fun(); } }
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9 days ago, # |
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Сan someone explain why this formula for calculating XOR(0;x) works? Why are we looking on x (mod 4)???

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5 days ago, # |
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For problem C, I'm getting AC on the one given in the tutorial, and TLE in the code, which I've written. Thing is, both codes are exactly the same except for the format for input taking. Someone see to it. AC — https://codeforces.me/contest/2036/submission/291926108 TLE — https://codeforces.me/contest/2036/submission/291925987 infact, this one too gives TLE — https://codeforces.me/contest/2036/submission/291924193

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    5 days ago, # ^ |
    Rev. 3   Vote: I like it +1 Vote: I do not like it

    In the check function, you are directly passing string s which results in a new copy everytime the function is called. Use pass by reference instead. Your Submission. The other one uses a global buffer.

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      4 days ago, # ^ |
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      Thanks a lot man, it really made me learn something new

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2 days ago, # |
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For problem E, why Complexity:O(n⋅k+q⋅m⋅logn) won't TLE, (q is 1e5 and m is 1e5)