### flamestorm's blog

By flamestorm, 2 months ago, Thanks for participating!

1703A - YES or YES?

Idea: flamestorm

Tutorial
Solution

1703B - ICPC Balloons

Idea: flamestorm

Tutorial
Solution

1703C - Cypher

Idea: mesanu

Tutorial
Solution

1703D - Double Strings

Idea: MikeMirzayanov

Tutorial
Solution

1703E - Mirror Grid

Idea: mesanu

Tutorial
Solution

1703F - Yet Another Problem About Pairs Satisfying an Inequality

Idea: flamestorm

Tutorial
Solution

1703G - Good Key, Bad Key

Idea: mesanu

Tutorial
Solution Tutorial of Codeforces Round #806 (Div. 4) Comments (163)
 » Quickly
 » Thanks for fast tutorial
 » Amazing contest !! ..E is tricky one!
•  » » It was exactly the same as rotating a 2d array using two for loops on leetcode.
•  » » That's why I Aced F instead of E.
•  » » Yeah, I just used the same logic(leetcode 90 degree rotation) to create all four rotated configurations of the matrix and then checked min moves to make any cell equal in all four configurations.
•  » » E is full implementation based
 » 2 months ago, # | ← Rev. 2 →   F is solvable with prefix sums in $O(n)$.For each $i$ count the number of elements, where $j \leq i$ and $a_j •  » » You beat me by 1 second, gonna say the same thing. •  » » » 2 months ago, # ^ | ← Rev. 6 → •  » » can you please explain a bit more?the prefix sum approach isn't really clear to me? •  » » » For every element we can store how many element we have encountered before it, that have v[i] < i int c = 0; for(int i = 0; i < n; i++) { cnt[i] = c; // 1 Based Indexing c += (i + 1 > v[i]); } Now for every element we can find whether we have it's value in range and whether we can find element so that we can satisfy the 2nd part in the condition i.e. i < v[j], other conditions are already satisfied int ans = 0; for(int i = n - 1; i >= 0; i--) { // We check for an element only when the element itself follows the v[i] < i rule // AND // Only when its index is in our range 1 to n-1 (skipped 0 because there are no elements before it) // THEN // we find how many elements are before the index equal to i since there is strict inequality if (i + 1 > v[i] && v[i] > 1) ans += cnt[v[i] - 1]; } My Submission •  » » » » woww great idea understood completely •  » » » 2 months ago, # ^ | ← Rev. 2 → the number of pairs$i != j $of n elements is$C_{n}^{2}=\frac{n(n-1)}{2}=\sum_{i = 1}^{n-1}i $so you can keep a prefix sum of number elements that satisfied the condition$i>a_i$•  » » Can you plz tell me why your code is giving correct ans for the case when a[i]-1 becomes -1. Beacause that should give a garbage value •  » » » My bad, thanks for noticing. Probably it is$0$in this case. Now the code is correct. •  » » 2 months ago, # ^ | ← Rev. 6 → i've solved it Using ordered Multiset:after Getting a value which is lesser than its index just check how many values are present in the set which is lesser than this...and after this just insert the index of the value ( not the value! to satisfy ""ai •  » » Can G be solved by DP ? •  » » » Yes I solved G using DP. I used the fact that applying the bad operation 30 times would make all elements 0 since log(1e9) is approximately 30. Here's my solution: https://codeforces.com/contest/1703/submission/163933460 •  » » » Yes, I made a Recursion + Memoization solution, using similar concept. Check it here : Recursive Solution •  » » Just wanted to comment the exact same thing. My solution •  » » :))  » I will say that it was a good contest. Thanks for the quick tutorial  » I'll just say it. This contest is the best for me .  » D and F are good problems.  » Hey! Can anyone clarify why my solution is showing TLE on Test Case 10 (Problem D), my approach is similar as that mentioned in the Editorial. Thanks! 163871330 •  » » 2 months ago, # ^ | ← Rev. 2 → Your problem lies here I suppose: Spoilerif(flag || mark[v[i]]){ ans=ans+'1'; mark[v[i]]=1; } else{ ans = ans+'0'; mark[v[i]]=0; } When you wrote ans = ans + '1' what the code did was calculate ans + '1' then assign that resulted string to ans, which made this operation$O(N)$. Use ans += '1' next time.Bonus: Here's your very same code accepted with those changes. •  » » » Thanks! •  » » 2 months ago, # ^ | ← Rev. 2 → You just need to speed up the I/O. Just add this line (before defining t int your main funtion). ios::sync_with_stdio(false);cin.tie(nullptr); After that, tabulate the code correctly and you'll get AC. •  » » » No it didn'tFast IO does not solve every problem. •  » » » » But your tabulation is not correct. •  » » » » » But who asked.  » 2 months ago, # | ← Rev. 2 → I think problem F can be done in O(n) time.As mentioned, we only need to consider indices i such that a_i < i. Call such indices "good". We can make a prefix array such that the ith element is the number of good indices less than or equal to i.Now consider all possible pairs (i,j) that work for a fixed good index j. The number of possible values for i is the number of good indices less than a_j, which is stored within the prefix array. The answer can then be found by summing across all good indices j.The prefix array can be created in 1 pass, so that takes O(n) time. The summing across all good indices can also be done in 1 pass, so this is O(n) time again.Code for this can be found in my submissionEdit: Apparently got sniped lmao •  » » Can you plz tell me why your code is giving correct ans for the case when a[i]-1 becomes -1. Beacause that should give a garbage value •  » » » I specifically excluded it in the if condition to handle that bug if a[i] < i+1 and a[i] > 0: •  » » 2 months ago, # ^ | ← Rev. 2 → Had a similar idea but instead of using a prefix array, for all indices i where a_i •  » » i have done the same. What do you mean by "sniped" ? •  » » » Someone else said a similar thing while I was typing my message  » 2 months ago, # | ← Rev. 2 → Anyone could tell me what's wrong with this approach?  » Problem G was a good one. •  » » Yes  » For the problem D we can use unordered_set cache to store the strings. Then for each string, bruteforrce divide the strings in two and check whether both of these strings exist in the cache or not. Simple •  » » to be honest though, you should be very cautious on using hash-based data structures in codeforces. Some people are masters of hacking and can easily find hundreds, even thousands of hash collisions. •  » » » Thats true... I think one way we can avoid is by making 2 or three hashes differently and storing in each of them. That way the chance of getting same hashes reduces significantly  » 2 months ago, # | ← Rev. 2 → This code of mine for question D(double strings) is giving TLE on test 3 and I don't know why. Please help me where is the problem? #include using namespace std; #define ll long long int char check_string(ll k,vector &v){ if(v[k].size() == 1) return '0'; for(ll i=0;i>t; while(t--){ ll n; cin>>n; vector v(n); for(ll i=0;i>v[i]; } string s; for(ll i=0;i •  » » Your solution is$O(n^2)$. •  » » use markdown correctly plz  » Can F be solved by Fenwick tree?? •  » » Yes. 163916062 •  » » » Yes •  » » 2 months ago, # ^ | ← Rev. 3 → any solution which can count pairs where$i < a_j$where$a_i < i, a_j < j$in all$i$and$j$is a valid solution. therefore Segment Trees and Fenwick Trees are valid solutions, so are prefix sums. •  » » Yes. I have done it using segment tree. here is my solution 163925757  » Honestly, F was too easy with Segment Trees (or Fenwick Trees, those two have identical purposes). I think any person with some experience of common segment tree problems would easily solve it. •  » » BS enters the room*Prefix Sum enters next and slap both on the face* •  » » it can be easily solved with pbds with a few lines of code •  » » yes, very easy code.  » Let's see in the hacking phase if people learned to use the map instead of the unordered_map. beethoven97 I summon you!  » 2 months ago, # | ← Rev. 2 → why this solution 163935126 giving TLE ? Anybody please help •  » » because memset function fills the DP using its size --> O(N), here it is N = 1e5 so your dp gets filled for all test cases which can be upto 1e4, so ~1e9 operations per second are performed. use thisfor(int i = 0;i <= n; i++){ for(int j = 0;j < 32; j++){ dp[i][j] = -1; } }  •  » » » Thank you •  » » you are initializing the dp array in each test case. There can be up to 1e4 test cases. Even if n is 1 in all test cases, you're initializing an array of size 3e6 in each test case. In the worst case, you'll be doing 3e10 operations which is way out of the problem constraints. To avoid this, you can declare a dynamic array(vector) and resize it in each test case according to the value of n. Because we're guaranteed that the sum of n across all test cases doesn't exceed 1e5 •  » » » Thank you. •  » » You can try something like this.163938289  » Thanks for this amazing contest and fast tutorial.  » 2 months ago, # | ← Rev. 2 → why my dp solution doesn't work for G https://codeforces.com/contest/1703/submission/163929073 •  » » For the testcase: 1 32 100 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 your code gives a negative answer. The dp needs to be able to go from dp[i-1] to dp[i] without decreasing it by k. •  » » » Oh thats the problem...Thnx •  » » » Here is a little trick. We know, that answer always$0$or bigger (because we can just open chests for free). That means answer is not only$max(dp[n][i]) \; for \; all \; i < 32$, but$max(dp[i][j]) \; for \; all \; i < n \; all \; j < 32$ » thank you  » 2 months ago, # | ← Rev. 2 → Thank you for the very good contest, all the problems are enjoyableI learn some new thing from problem G  » This was the first contest I attempted. I really liked the problems even though I couldn't solve them. Thanks for all the effort. :D  » I've gained four ideas from issues I thank those who wrote those for issues and I hope that the contests div4 will increase in future  » can somebody explain probelm E again , what if 1 0 0 11 0 0 1 1 0 0 1 , if this is the test case , does it have zero movies required? •  » » The number of rows and columns should be equal in this problem. So your testcase is invalid. •  » » » sorry just consider this one 1 0 0 11 0 0 11 0 0 11 0 0 1 •  » » » » 4 moves are required. U have to make either top 2 zeroes and bottom 2 zeroes to one or u can either make left two ones and right two ones to zero.  » Amazing contest , I've gained four ideas from issues I thank those who wrote those for issues and I hope that the contests div4 will increase in future  » i tried to solve F with multiset (c++) along with lower_bound, distance methods here: linkbut i get TLE, why? •  » » distance has O(N) complexity for set e multiset (N is the size of the set) •  » » » 2 months ago, # ^ | ← Rev. 2 → is there a way around this to get O(1) distance operation? •  » » » » distance has O(1) complexity only with random access data structures such as vectors. check the complexity section: https://m.cplusplus.com/reference/iterator/distance/ •  » » » » » ok thanksnever doing that again •  » » » » » » I implemented it using 2 sets. Check my submission.  » 2 months ago, # | ← Rev. 2 → https://ideone.com/diXeOD (My code) Problem F using Ordered Set. Simple Code.. Hope it will help! •  » » 2 months ago, # ^ | ← Rev. 2 → okay, so order_of_key is like lower_bound? without iterator part? •  » » » "the number of items in a set that are strictly smaller than our item". you can see here https://codeforces.com/blog/entry/11080 •  » » » » ok got it, thanks everyone •  » » » 2 months ago, # ^ | ← Rev. 2 → It returns how many values are strictly lesser than this in order of log(n) time  » So amazing  » Can anyone tell why using vector in this solution for problem E gives wrong answer? Replacing vector with plain array makes it accepted. Submission link •  » » Look at the case when x = 0, you are accessing sum[x-1] or sum[-1] •  » » » You're right. I guess it was luck that it got accepted with c array.  » 2 months ago, # | ← Rev. 3 → someone hacked my D (163901816) can anyone provide me test case ? •  » » Wait for system testing •  » » ans = ans + '1' makes copy of ans then adds '1' you should use ans += '1'  » Why 163895211 is giving TLE?  » 2 months ago, # | ← Rev. 4 → DP solution for G. Lang: PyPy3-64 ____________________________________________________________________________________ t = int(input()) for _ in range(t): n, k = map(int, input().split()) *arr, = map(int, input().split()) MAX_HALF_CNT, NINF = 1, -2**63 max_coins = max(arr) while max_coins >> MAX_HALF_CNT: MAX_HALF_CNT += 1 MAX_HALF_CNT += 1 dp = [[NINF] * MAX_HALF_CNT for _ in range(n)] # dp[i][half_cnt] 打开第i个抽屉后所能得到的最大硬币数，其中使用了half_cnt次坏钥匙， dp = arr - k # 用好钥匙，减k dp = (arr >> 1) # 用坏钥匙，减半 # print(dp) for i in range(n-1): for h in range(MAX_HALF_CNT): dp[i+1][h] = max(dp[i+1][h], dp[i][h] + (arr[i+1] >> h) - k) if h + 1 < MAX_HALF_CNT: dp[i+1][h+1] = max(dp[i+1][h+1], dp[i][h] + (arr[i+1] >> (h + 1))) else: # 啥都没了 dp[i+1][h] = max(dp[i+1][h], dp[i][h] + 0) print(max(dp[n-1]))   » The idea for solving Problem F was very nice  » Could help me G problem?i use dp to slove it ,but i cant fine my problem.thank u very muchproblem GTAT...my english is poor ,but i try to share my ideaj:the number of good keypw(2,x)=2**xwhen we open the i th box ,if we use j good keys so we use (i-j) bad keys.but pw(2,33)>1e9,so i think the number of good key is lower than 33.if the number of bad key is upper than 33,we can see it as 1e14.so when we open some box,if we use good keys ,we got a[i]/(pw[(i-j)] -m cost and we use bad keys ,we got a[i]/(pw[(i-j)] -mso DP[i][j] is mean open i boxes use j good keys.Dp[i][j]=Dp[i-1][j]+ bad keys or DP[i][j]=DP[i-1][j-1] + good keysso whats wrong with me TAT •  » » Suppose that n and k are very large And the optimal operations are always use bad keys. In this case obviously we use more than 32 bad keys but your solution don't considers this and gives the wrong answer.To fix it : for calculating answer iterate over all dp values and get maximum instead of only dp[n — 1] (because if we do this we consider above case): for(int i = 0; i < n; i++) for(int j = 0; j < 32; j++) ans = max(ans, dp[i][j]); My submission : 163915709 •  » » » I am getting TLE on second TC, can you ples help LINKThanks. •  » » » » i think you need let done be 31 •  » » » thank u ,i know  » 2 months ago, # | ← Rev. 3 → Can anyone help me find why I'm getting TLE on 5th case and how to further optimize this code? #include using namespace std; vector> dp; long long rec(vector& arr, long long k, long long factor, long long i){ if(i>=arr.size()) return 0; if(dp[i][factor]!=-1) return dp[i][factor]; long long temp=-k+arr[i]/pow(2, factor)+rec(arr, k, factor, i+1); long long t=arr[i]/pow(2, factor+1)+rec(arr, k, factor+1, i+1); if(temp>t) return dp[i][factor]=temp; return dp[i][factor]=t; } int main(){ ios::sync_with_stdio(false); cin.tie(nullptr); long long t; cin>>t; while(t--){ dp.clear(); long long n, k; cin>>n>>k; vector arr(n); for(long long i=0;i>arr[i]; dp.resize(n+1, vector(n+2, -1)); cout< •  » » link There you go, Its working nowI changed your dp size n , 32 and early pruning in base case •  » » » Thanks a lot man.  » For which test case it fails.. My solution  » For first 30 minutes i think in E we need to make matrix by add 1. Later i eays accepted this Problem  » Hey! Can someone please clarify why my Dp solution for Problem G is showing TLE on Test Case 3? Thanks!163941930  » 2 months ago, # | ← Rev. 2 → G is quite hard, other problems are great. Hope the rating will change soon so well-perform participants could get to the next rank!  » I did a,b,c,d,e,f with 0 wrong submission can i expect pupil ... Now  » Thanks for the fast editorial  » 2 months ago, # | ← Rev. 2 → Another F approach. We can create p where p[i] = (a[i] < i), after that create a prefix sum on that array and iterate over a and do the following: check if p[i] = 1, otherwise continue check if a[i] >= 2, otherwise continue add prefsum[a[i] — 1] to the answer So why do we need a[i] >= 2 statement? If a[i] < 2, then there is no way to satisfy this inequality. a[i] >= 0, i >= 1, therefore a[i] < i < 1 < j can't be satisfied. Check out my implementation if you are interested 164005949. What do you think about this one?P.S Well, just find out some guy already told about the same approach...  » Simple recursive DP solution for G here.  » map? Why not just use a set?  » 2 months ago, # | ← Rev. 2 → Can anyone help to reduce time complexity of G #include using namespace std; long long arr; long long rec(long long i,long long n,long long p,vector v,long long k,vector>& dp){ if(i==n || p>30) return 0; if(dp[i][p]!=-1) return dp[i][p]; int x = v[i]/arr[p]; return dp[i][p] = max(x-k+rec(i+1,n,p,v,k,dp),x/2+rec(i+1,n,p+1,v,k,dp)); } int main() { long long t;cin >> t; arr = 1; for(long long i=1;i<32;i++) arr[i] = 2*arr[i-1]; while(t--){ long long n,a,ans=0,k; cin >> n >> k; vector v(n,0); vector> dp(n,vector(32,-1)); for(long long i=0;i> v[i]; cout << rec(0,n,0,v,k,dp) << endl; } return 0; }   » Can someone tell me why this dp solution is getting WA, doesn't make any sense to me? https://codeforces.com/contest/1703/submission/163945776 •  » » You also have to update your answer while calculating dp so as to counter cases when you use more than 30 bad moves. One such example will be, 60 chests containing 2 coins and lets say k = 1e9. •  » » » You are saying that i have to take the max of the whole matrix? •  » » » » Not really the whole matrix, max of$dp[i]$over all chests$i$+ your current logic will be sufficient. •  » » » » » Okay thanks, i fixed it •  » » 2 months ago, # ^ | ← Rev. 2 → not max { dp[n][i] },try max{ dp[i][j]} because maybe after dp[i][j] all cost of box if 0.so maybe this i is not n.  » Video Solutions of complete problemset.  » When is the result? •  » » did ur rating update? i am newbie but rating not updated •  » » » No , i am waiting  » F can be solved in linear time. submission  » What is the rating range for problems F and G?  » hey my rating is not updated even I am a newbie and gave 8-9 rated contests before  » Ratings did not seem to update. Anyone else having this problem? •  » » Yes... Not updated •  » » » Just checked again. Ratings are updated now.  » Can anyone please tell why am I getting Memory Limit Exceeded in Problem E? I am just using a 2D Array. Link to my submission :- https://codeforces.com/contest/1703/submission/163908463 •  » » Hello sir, There are two layers of recurrence in your code, so your code's time complexity is$O(n^2)$, and when$n = 200000$,you will get TLE. I solved this problem with set, and my time complexity is$o(n \log n)$.This is my code: # include #define ll long long #define re register #define il inline using namespace std; string s; int main() { int t; scanf("%d", &t); while (t--) { int n; bool f = 0; scanf("%d", &n); set str; for (int i = 1; i <= n; ++i) { cin >> s[i]; str.insert(s[i]); } for (int i = 1; i <= n; ++i) { f = 0; for (int j = 1; j < s[i].size(); ++j) { string a = s[i].substr(0, j), b = s[i].substr(j, s[i].size()); if (str.find(a) != str.end() && str.find(b) != str.end()) { printf("1"); f = 1; break; } } if (f == 0) printf("0"); } printf("\n"); } return 0; } Although there are two lawers of recurrence in my code, but$s[i].size \le 8$, so my code doesn't go TLE. •  » » » Hey, Thanks for the reply but I am asking about Problem E, Mirror Grid. The solution that you have explained is Problem D, Double Strings. •  » » » » Sorry, it's my mistake. Let me show you my code of problem E. # include #define ll long long #define re register #define il inline using namespace std; char g; int f; int cnt; int main() { // freopen("output.out", "w", stdout); int t; scanf("%d", &t); while (t--) { int n, ans = 0; scanf("%d", &n); for (int i = 1; i <= n; ++i) { string s; cin >> s; for (int j = 1; j <= n; ++j) g[i][j] = int(s[j - 1] - '0'); } int len = (n + 1) >> 1; for (int i = 1; i <= len; ++i) { int k; if (n % 2) k = len - 1; else k = len; for (int j = 1; j <= k; ++j) { cnt = cnt = 0; int x = n - i + 1, y = n - j + 1; cnt[g[i][j]]++; cnt[g[j][x]]++; cnt[g[y][i]]++; cnt[g[x][y]]++; // printf("%d %d %d %d\n", i, j, cnt, cnt); ans += min(4 - cnt, cnt); } } printf("%d\n", ans); } return 0; }  •  » » You are creating new 2D array every test case. •  » » » Thank you so much for replying. Can you tell what should be approach for this problem? •  » » » » To be honest I didn't solve this problem and now I don't have time now because the contest starts in 2 minutes.To fix your problem (MLE) you can create 2D array before first test case. (before while (t > 0)` line) •  » » » Because in the editorial they are also creating a 2D array for each test case. •  » » » » Then I don't know. Maybe I was wrong.  » 2 months ago, # | ← Rev. 2 → Loved the round! Wish I could solve G. Thanks to the setters and testers.  » I solved F in O(n) Here is my solution: 163897273  » Why also use map mp in D,but i Time limit exceeded.  » 2 months ago, # | ← Rev. 2 → i did question f in O(n logn) but still it is showing me TLE in one test case can anyone help me out please .... i would be grateful my sol 164096201 •  » » you have to pass the vector by reference in the find_pos function  » C no Problem is quite interesting! in my opinion  » Good solution for problem G!  » For question good keys bad keys why is recursion + memo wrongmy submission 164141664 •  » » 2 months ago, # ^ | ← Rev. 2 → figured out the solution. made a silly mistake. Ignore the above comment •  » » 2 months ago, # ^ | ← Rev. 2 → I did the same recursion + memo. check here: Recursion+Memo  » Note that, in problem G, don't use memset all the times, or you will get TLE on test 2 (I had tried it), because memset has a time complexity as$O(n \times \log a_i)$, if$t\$ is very big, the total time complexity will be too large to solve this problem.I hope that this note will help you!
 » I am getting runtime error in G. Please help me out. Here is my submission
 » Can someone suggest more problems similar to G?(like DP states like this)
 » good round
 » good
 » Good Tutorial!
 » Can someone please explain to me what is wrong with my solution for F or provide a counter example? I probably messed up my binary search implementation but cannot figure it out. https://codeforces.com/contest/1703/submission/165016289
 » 2 months ago, # | ← Rev. 2 →   anyone please tell me my mistake in problem G 165412847
 » Call a pair good if it satisfies the condition. Let's split the inequality into three parts: ai
 » I intuitively got the solution for G but was unable to prove it. The proof in the editorial is great!
 » 2 months ago, # | ← Rev. 4 →   https://codeforces.com/contest/1703/submission/165888535I solved the question Double strings using a map but I still didn't get how the time complexity is O(l*nlogn) According to my code time complexity would belogn->to insert elements in mapn->outer loop l->maximum length of input string l->substr(inbuiltfunction) Total TC->logn+n*(l*l)->(l^2)*(n)
 » lol, D is copy of https://acmp.ru/?main=task&id_task=87