### 351F44's blog

By 351F44, 2 years ago, Problem A is developed by mejiamejia.

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#### 1733E - Conveyor

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Challenge Tutorial of Codeforces Round #821 (Div. 2) 1733, Comments (81)
 » Can someone explain/share the O(N) solution of task D2?
•  » » If $x \geq y$, follow D1. Otherwise, for $x < y$, observe that for each mismatch, we can fix it by either performing a single $y$ operation with another non-adjacent mismatch, or we can perform a sequence of $x$-operations to either the next or previous mismatch. We can use a 1D DP as follows:$dp[i]$ represents the minimum cost for dealing with the first $i$ mismatches alone. If $i$ is even, then all mismatches should be fixed. If $i$ is odd, then $i - 1$ mismatches should be fixed while one mismatch remains. The $mis[]$ array stores mismatch indices.For even $i$, $dp[i] = \min (dp[i - 2] + (mis[i] - mis[i - 1])x, dp[i - 1] + y)$. We can fix the $i$-th mismatch through either an $x$-operation chain with the $(i - 1)$-th mismatch (first option), or through a $y$-operation with some earlier mismatch (second option).For odd $i$, $dp[i] = \min (dp[i - 2] + (mis[i] - mis[i - 1])x, dp[i - 1])$. Here, for the $i$-th mismatch, the first option is the same as before ($x$-operation chain with $(i - 1)$-th mismatch), but the second option is to let the $i$-th mismatch be the unresolved one. Note that $dp[i - 1] \leq dp[i - 2] + y$ (see even formula), so there is no need to consider using a $y$-operation for the $i$-th mismatch for odd $i$ (but it wouldn't change the result if this was added as a candidate). The last element in the $dp$ array (which must be an even index) is the answer.
•  » » » This is same as tourist solution.
•  » » » This solution looks more elegant!
•  » » » Very well explained, I understood it without trouble. This post should be in the editorial.
•  » » » Better than the editorial. Thanks!
•  » » » How would you add the the y-operation for odd i, if we wanted to? Also please can you explain a little why there is no need to consider the y-operation in case of odd i?
•  » » » » If you wanted to consider the $y$-operation for the last element of odd $i$, it would have a cost of $dp[i - 2] + y$ (so the second-to-last element is the unresolved one). However, we know that $dp[i - 1] \leq dp[i - 2] + y$ (because applying the even formula on $i - 1$ yields $dp[i - 1] = \min (dp[i - 3] + (mis[i - 1] - mis[i - 2])x, dp[i - 2], dp[i - 2] + y) \leq dp[i - 2] + y$). Therefore, for odd $i$:$dp[i] = \min (dp[i - 2] + (mis[i] - mis[i - 1])x, dp[i - 1], dp[i - 2] + y) = \min (dp[i - 2] + (mis[i] - mis[i - 1])x, dp[i - 1])$
•  » » » » » Thanks for replying! But I still have one doubt... for y-operation for last element of odd i, why have you only tried — by keeping either last element unresolved or second last element unresolved? Why didn't you try keeping any other element unresolved?
•  » » » » » » All other options are implicitly covered by $dp[i - 2]$, which denotes the optimal way of handling the first $i - 2$ elements. I should correct my earlier statement actually: $dp[i - 2] + y$ does not only cover the case where the second-to-last element is unresolved, but it actually covers the cases where any of the first $i - 1$ elements are unresolved. However, note that the different options is not about "which element is unresolved" but more about "how do we deal with the last element". We can break this down to four options: Don't pair the last element: this costs $dp[i - 1]$. It would, of course, also be the unresolved element in this case. Pair it with the second-to-last element using an $x$-operation chain: this costs $dp[i - 2] + (mis[i] - mis[i - 1])x$. Here, the unresolved element is whoever was unresolved in $dp[i - 2]$ (which is the optimal choice among the first $i - 2$ elements). Pair it with the $(i - 1)$-th element using a $y$-operation (assuming they're not adjacent). The cost here is $dp[i - 2] + y$. Again, the unresolved element is whoever was unresolved in $dp[i - 2]$. The assumption of non-adjacency is not an issue, because if the last two elements were adjacent, then option 2 would win over option 3 (since $x < y$). Pair it with one of the first $(i - 2)$ elements using a $y$-operation. The optimal way to deal with the first $(i - 2)$ elements is given with $dp[i - 2]$, which includes one unresolved element among them. Therefore, the optimal way to handle this case would be to pair the last element with whoever is unresolved from $dp[i - 2]$. This does mean that the $(i - 1)$-th element is not handled, so it would end up becoming the unresolved one. The cost here is also $dp[i - 2] + y$. If the fourth option still confuses you, let me elaborate a little more: we are not specifically choosing that the $(i - 1)$-th element is the unresolved one, but rather, we are choosing to let the last element pair with some element from among the first $(i - 2)$ elements. The optimal choice here is to pair it with the unresolved element in $dp[i - 2]$. [Proof: if it's better to pair it with some different element $j$, this would imply that leaving $j$ unresolved would be better for the value of $dp[i - 2]$ than whichever element was actually left unresolved]. And this would naturally lead to the $(i - 1)$-th element being the unresolved one, since all other elements are handled. In reality, there is no actual difference between options 3 and 4. They can both be combined into a single case of "use a $y$-operation on the last element". With an understanding of how a $y$-operation works, where the cost is the same irrespective of which element we pick (as long as it's non-adjacent, but option 2's superiority would dominate such scenarios), it should be intuitive to observe that it doesn't actually matter as to which element we are pairing it, a long as it's a $y$-operation. Finally, given that the unresolved element would eventually have to be resolved in later even cases, either through an $x$-operation chain with the next element (which requires that the last element is the unresolved element) or a $y$-operation with some element (where there is no advantage for any particular element over another), it's easy to see that keeping the last element unresolved cannot be worse than to pair it up with a $y$-operation so that someone else can wait for a future $y$-operation, i.e., option 1 will never be worse than options 3 or 4.Sorry for the wall of text; I think this scenario is actually really simple intuitively, but I can understand that one might have doubts about whether this intuition is rigorously proven, so I hope I was able to ease some of those doubts.
•  » » » NB
•  » » 2 weeks ago, # ^ | ← Rev. 4 →   There is a solution with an $O(n)$ DP.For $x\ge y$ the solution is obvious (it's Problem D1), so I'll assume that $x •  » » 2 weeks ago, # ^ | ← Rev. 2 → Thanks for the explanation.I have seen another tricky solution: We will exclude the x>=y as we can solve it with D1 solution and the case where only 2 nearby mismatches( ans=min(2*y,x) ),(Option#1): So you will try to jump to any other mismatch with cost Y and goes to the next mismatch (dp[i] = dp[i+1] + y), I think this is valid because if you will take any mismatch with cost Y you will pair it with another mismatch.The problem with option 1 is calculating Y for every mismatch, so you have to count the number of mismatches taken by the first option and divide them by 2 , to avoid that, you can just multiply the second option (x) by 2 and divide the final solution by 2.(Option#2): The other transition is to chain with the next one(i+1) and transition to (i+2) so you solved i and i+1 mismatches with cost x*(mismatch[i+1] — mismatch[i+2]). option 2 is only valid if there exists (i+1 •  » » 2 weeks ago, # ^ | ← Rev. 2 → let v be the sorted vector of indices where a[i]!=b[i] (let size of v be vn) then the answer of the problem is f(0)where, cost(i,j)=min(y,|v[i]-v[j]|*x)f(vn-1)=y,f(cn-2)=cost(vn-1,vn-2)f(i) = min(((vn-i)%2)*y + f(i+1),cost(i,i+1)+f(i+2))  » was this blog in 'draft' for 2 years? •  » » Yes.  » Hello Guys! For D2 I had this solution 172748609, I also don't no the reason that why its true.Can any one hack or say the reason behind it? •  » » 2 weeks ago, # ^ | ← Rev. 2 → First, we get all the positions which satisfy a!=b and save them in a new array C. Then we consider four positions which are adjacent in C. Let them be p1,p2,p3,p4 (p1  » 2 weeks ago, # | ← Rev. 4 → Why do you say that a greedy solution does not work in D2. I think it's too bad that such tasks appear in the competition! •  » » Edited the phrase more clearly. •  » » LoL, what did u expect from the fifth problem? •  » » » 2 weeks ago, # ^ | ← Rev. 2 → I'm pretty sure the post was to be interpreted with a "/s", considering the "not so unusual" hint (as it was before 351F44 edited it).  » 2 weeks ago, # | ← Rev. 3 → Hello guys, would really appreciate a little help with the B question. I thought that if we can make a combination with the 2 values of x and y that sums up to length-1 then we will get the right answer(was not sure so just tried). So I ran binary search between 0 and length-1 to search for the number of players that had x wins to find the answer and then print the player x times, but I got WA. here is the code:https://codeforces.com/contest/1733/submission/172718129 kindly guide me, thank you. EDIT:I found the error,it was a silly mistake in the printing part.THe code is working fine now. Code:https://codeforces.com/contest/1733/submission/172869494 •  » » Binary search only works on monotonic searching space. Simple words, if x works either y > x or y < x should work. I am not sure how this problem is related to binary search. Maybe explain why you think binary search will do here. •  » » » thank you for replying.Actually as i said in the post,I worked on the logic that if i can find a value k such that (k*x)+((length-k)*y) can form length-1(as we will have total wins as length-1) then we will have an answer. So the value of k can range from 0 to length-1. therefore to find that k i used binary search. I know I many have made some mistakes,therefore any tips are appreciated. Thank you •  » » » Thanks a lot for your help. I had made a minor error in my code in the printing the other logic was correct. The solution do works correctly now. Code:https://codeforces.com/contest/1733/submission/172869494 •  » » Take a look at Ticket 16198 from CF Stress for a counter example.This should help you figure out why binary search wouldn't work. •  » » » Thanks a lot for your help. the code had a minor error in the printing the other logic was correct. Here is the solution Code:https://codeforces.com/contest/1733/submission/172869494 •  » » » » You got it!  » Does anyone have a solution for D2 using DP with memoization? •  » » You can check my solution: 172731715, The main part:void Process() { ll n, x, y; cin >> n >> x >> y; string a, b; cin >> a >> b; vec C; FOR(i, 0, n - 1) { if (a[i] != b[i]) C.pb(i); } ll s = C.size(); if (s % 2 == 1) { cout << -1 << '\n'; return; } if (x >= y) { ll ans = y * (s / 2); if ((ll)C.size() == 2) { if (C == C + 1) ans = min(x, y * 2); else ans = min((C - C) * x, y); cout << ans << '\n'; return; } if ((ll)C.size() == 0) { cout << 0 << '\n'; } else { cout << (s / 2) * y << '\n'; } return; } FOR(i, 0, s) { FOR(j, 0, s) { DP[i][j] = -1; } } function dp = [&](ll l, ll r) { if (DP[l][r] != -1) return DP[l][r]; if (l == r) return DP[l][r] = 0; if (r == l + 1) { if (C[r] == C[l] + 1) return DP[l][r] = min(x, y * 2); return DP[l][r] = min((C[r] - C[l]) * x, y); } DP[l][r] = 1e18; DP[l][r] = min(DP[l][r], dp(l, l + 1) + dp(l + 2, r)); DP[l][r] = min(DP[l][r], dp(l + 1, r - 1) + y); DP[l][r] = min(DP[l][r], dp(l, r - 2) + dp(r - 1, r)); return DP[l][r]; }; cout << dp(0, s - 1) << '\n'; }  •  » » 2 weeks ago, # ^ | ← Rev. 2 → you can see my solution of D2 using recursion+memoizatoin: https://codeforces.com/contest/1733/submission/172789187 •  » » Let us say we have a vector of mismatched indices. We will be picking from this vector in pairs. In the DP, at any mismatched index, we have following options: We can pair this index with the next mismatched index by continuously changing everything in between. Cost would be distance between mismatched indices multiplied by x. We can pair with any previously unpaired index. Cost would be y. We don't pair with anything, just create another unpaired index. It will need to be paired with later. Cost is 0 as we have not resolved anything. Here is my submission https://codeforces.com/contest/1733/submission/172860412 •  » »  » In D2(editorial), how is it possible to have$i+1$ones in the first$i$elements of$c$? •  » » i is correct. Now edited. Thanks for noticing me. •  » » » I think there is one more problem with it.You recalculate dp for all$0 \le j \le i$, but in formulas where$c_i=1$there is a condition$j=i+1$. •  » » » » Edited, thanks. There was some mistake while translating 0-based MCS into 1-based editorial.  » What would be the expected rating of the first 2 questions? •  » » We estimated A as *800, B as *1000 or *1100. Let's wait. •  » » » and C? •  » » » » I think *1500. •  » » » » » Are you sure? I normally can't solve 1500s but C and even D1 were pretty solvable for me •  » » » » » » Same, I was guessing around 1300. •  » » » » » » » Well, standing says it can be easier than *1500. It is what I estimated before the contest, maybe real difficulty is more easier? :/ •  » » » » » » » » Yeah could be. Thank you for the contest tho! Great problems.  » Great Problem E! Though its implementation isn't complicated, its idea is quite thought-provoking and requires insight. This is how CF problems must be. Truly nice problem!  » If anyone feel that the editorial is too complex and you have solved longest palindromic subsequence or longest palindromic substring or https://codeforces.com/contest/1728/problem/DThen see my submission for problem D: https://codeforces.com/contest/1733/submission/172855405Very easy to understand and O(N*N) DP •  » » O(N) DP solution using Memoization: https://codeforces.com/contest/1733/submission/172865814Idea is to calculate how much we can save if we take or don't take x. Then subtract it from max cost possible i.e; y*(n/2)-dp[n]  » My code for problem D2 is giving Time Limit Exceeded at test case 3, Please help me in optimising my Approach, suggest me with this intution only, please help. ll f(ll i, string s, string s1, ll n, ll x, ll y,map&dp) { if(s==s1) return 0; if(i>=n) return 1e10+7; if(dp[s]!=0) return dp[s]; if(s[i]==s1[i]) return dp[s]=f(i+1, s, s1, n, x, y,dp); ll p=(ll)1e10+7, q=(ll)1e10+7; for(int j=0;j>n>>x>>y; string s,s1;cin>>s>>s1; mapdp; ll ans = f(0, s, s1, n, x, y,dp); if(ans>=(ll)1e9+7) cout<<-1; else cout<  » hats off...!!  » Is the challenge for E solvable? •  » » I don't know. •  » » 2 weeks ago, # ^ | ← Rev. 2 → I think it's plausible. Consider the state of the conveyer after using exactly k slimes and look at each diagonal, regarding right arrow as 0 and down arrow as 1. For example, when k = 0, all conveyers have right arrows, so all diagonal's values are 0. When k = 1, all conveyers except the first row have right arrows, so all diagonal's values are 1.As k increases, 1st diagonal's values are: 0, 1, 0, 1, ....2nd diagonal's values are: 00, 01, 11, 10, 00, ...3rd diagonal's values are: 000, 001, 011, 001, 101, 100, 110, 100, 000, ...4th diagonal's values are: 0000, 0001, 0011, 0111, 0011, 0001, 0011, 0111, 1111, 1110, 1100, 1000, 1100, 1110, 1100, 1000, 0000, ...It's not something I really proved, but it seems there's an obvious pattern here. I guess this pattern will allow us to find a path for k-th slime in O(N log k) for a N*N conveyer system.  » I'm curious to know how to solve problem c challenge, any hints ?  » A video editorial explaining the dp cases of D2 would be very helpful. Or maybe if someone can explain the cases here?  » I think stress plays a lot of role during contests. I had a whole 1 hour and 22 mins. left to do problem C during contest but was not able to do it, today tried to upsolve it and solved it in 15 mins without looking at the editorial.  » Please, share solution for Problem C challenge. Thanks. •  » » 2 weeks ago, # ^ | ← Rev. 2 → You can always make the whole array equal to the last element of the array in n-1 steps.First, apply the operation on the first and the last element. After this the first and the last element will become equal.Now just iterate through the array starting from the second element:Case 1. If the current element has the same parity as the last element apply operation to the current and the last element. This will make the current element equal to the last.Case 2. Otherwise, apply the operation to the current and the previous element. This will make the current element equal to the previous. But we know that the previous element is already equal to the last because we already iterated through it, so this will make the current element equal to the last. •  » » » Oh! I see. So the editorial's solution of n-1 operations is the optimal solution. Thanks. •  » » » This is the same solution as the orginal solution , however he asked about the solution of the challenge which asks to find minimum number of operations Test Sample : 2, 1, 3 according to your solution will cost 2 operations while the minimum cost is 1 operation we can apply the operation on second and third elements  » Difference between E's solution I submitted in last two minutes and the AC solution » Hello, 351F44 can you please tell in the question D1 & D2, how you even have a intuition of starting with the XORs of both the binary strings a and b. At my last thought after so long I could only think of working on b trying making equalto a. But actually of no use because it's the same thing. So how you even got the intuition of working on XORs of both the strings?? •  » » It is one of the result of an observation. To do such observation, check if it can be transformed to be structurally identical.  » There was an incorrect solution get accepted on D2 during the contest, and unfortunatly didn't get hacked. The idea was use range(or interval?) DP with some strange strategies to enumerate the decision. Code linkHope to strengthen the data soon.  » 172746684 else cout << min((r — l) * x, y) << endl; isn't it redundant to check for the minimum?  » any Recursive approach for D2?  » A typoNow, find out which cell contains clime ball among the diagonal. •  » » Edited. Thanks!  » For the problem E, is it possible to find out the periodicity of a particular roller in$O(1)$time or precompute them after which we can simulate the process for the slime that arrived at$t-x-y$in$O(x+y)\$ ?I don't have a concrete idea yet on how would one calculate the periodicity tho :( Any suggestions would be helpful! I feel one could exploit this to solve the challenge as well?
•  » » So I tried to print the pattern of the state of each roller form t = 1 till t = 50 and 0 represents right and 1 represents down and in the beginning between {1,0} and {0,1} you see a good pattern. but in the diagonal (x+y = 2) There doesn't seem to be a pattern that is being followed, but there seems to be some similarity in {0,2} and {2,0} perhaps but {1,1} seems to show no similarity ? And when we try talking about the diagonal (x+y = 3) I can't spot any good pattern. So I'm skeptical of being able to find a pattern in the periodicity although I'm almost certain that the states of all rollers are periodic in nature. Pattern in the state of each rollerEnter the coordinate 0 0 01010101010101010101010101010101010101010101010101 Enter the coordinate 0 1 00110011001100110011001100110011001100110011001100 Enter the coordinate 1 0 00011001100110011001100110011001100110011001100110 Enter the coordinate 1 1 00001000100010001000100010001000100010001000100010 Enter the coordinate 2 0 00000011110000111100001111000011110000111100001111 Enter the coordinate 0 2 00011110000111100001111000011110000111100001111000 Enter the coordinate 3 0 00000000000111111110000000011111111000000001111111 Enter the coordinate 1 2 00000111011110001000011101111000100001110111100010 Enter the coordinate 2 1 00000010001111011100001000111101110000100011110111 Enter the coordinate 0 4 00000111111111111111100000000000000001111111111111 Enter the coordinate 2 2 00000001100111001100000110011100110000011001110011 Enter the coordinate 
 » At E, I could think everything in time but the most important part, the simulation with x slime balls processes. How can I approach that process from the hints?
 » I wonder for D2, if we can just use DP to solve all the conditons? So if x>y, can we use DP instead of greedy algorithm? I try it, but got wrong answer. #include using namespace std; const int N=5010; typedef long long ll; ll z0[N][N],z1[N][N],c[N]; int main() { ios::sync_with_stdio(0),cin.tie(0),cout.tie(0); int _; cin>>_; while(_--){ int n,x,y; cin>>n>>x>>y; string a,b; cin>>a>>b; for(int i=1;i<=n;++i) c[i]=a[i-1]^b[i-1]; for(int i=0;i<=n;++i) fill(z0[i],z0[i]+n+1,1LL<<60); for(int i=0;i<=n;++i) fill(z1[i],z1[i]+n+1,1LL<<60); if(c) z1=0; else z0=0; int d=min(x,y); for(int i=2;i<=n;++i) if(c[i]){ for(int j=0;j<=i;++j){ if(j) z1[i][j]=min(z0[i-1][j-1],z1[i-1][j-1]),z0[i][j]=min(z0[i-1][j-1]+d,z1[i-1][j-1]+y); z0[i][j]=min(z0[i][j],z1[i-1][j+1]+d),z0[i][j]=min(z0[i][j],z0[i-1][j+1]+y); } } else{ for(int j=0;j<=i;++j){ z0[i][j]=min(z0[i-1][j],z1[i-1][j]); z1[i][j]=min(z0[i-1][j]+y,z1[i-1][j]+d); if(j>=2) z1[i][j]=min(z1[i][j],z1[i-1][j-2]+y),z1[i][j]=min(z1[i][j],z0[i-1][j-2]+d); } } if(z0[n]!=1LL<<60) cout<
 » if the problem C ask us that they want to minimize the number of operation , how we can solve it ?
 » Can anyone explain that in question B Rule of League, in sample test case 4 ans is 2 but it can also be 1 right. As this was stated there Each player has either won x games or y games in the championship. i can't understand it plz help. 5 5 2 0 8 1 2 3 0 0 2 0 1 6 3 0 
•  » » 9 days ago, # ^ | ← Rev. 7 →   yes , in test case 4 it can be 1 or 2 , both of them are correct . one of them won just one times and the second one won 0 times
•  » » » 173387621 I have tried it but they say its wrong this is my submission.
•  » » » » 9 days ago, # ^ | ← Rev. 2 →   if(mi != 0) cout<<-1<
•  » » » » » Thanks man that's a really rookie mistake.
 » Shout out for problem E. You broke CF's top rated member, and it took me a full day to solve it, well done
 » can some one explain this test case of problem D1 easy version4 8 3 01100000according to me the ans should be 8 but ans coming 6 Why?