[submission:https://codeforces.net/contest/1082/problem/problem:1082F]↵
↵
Hi. I will explain my approach to solve this hard and counter-intuitive problem.↵
↵
↵
**The problem**↵
------------------↵
↵
You've got some telephone numbers. Each number is dialed a given amount of times.↵
You would need to do an amount of key presses to dial all numbers, that is well defined.↵
However there is an amount of special buttons that can be used to type several digits at once. The buttons can only be used at the start of a number (before manually typing anything). Buttons presses does not count as digit press. Buttons can only be used once per number.↵
You need to decide how to place $k$ buttons to minimize total digit presses.↵
↵
↵
** First insight **↵
↵
We need to build a trie structure of the numbers, this will allow us to exploit numbers that have something in common↵
↵
)/predownloaded/ff/29/ff292827692d72106bcc201f94c5123a3a081d96.png)↵
↵
**Need of DP**↵
↵
We should note some kind of DP should be used using sub-trees. The sub-problem could be defined like↵
↵
"Solving sub-tree rooted at $i$ using $j$ buttons on strings belonging to that sub-tree.↵
↵
But there is a technical problem defining DP like that. Although it is logically valid to think "solving" as "assigning buttons" to strings on that sub-tree, we need to store some number to represent that way of "assigning buttons" on DP_{ij}.↵
The natural value would be "total key presses" to solve the sub-tree $i$ using $j$ buttons. But think about the characters of strings belonging to sub-tree but that are outside it. ↵
Should we consider it on sub-problem value? Yes or No?↵
↵
I have though about that fact some time and realized that we can't solve the problem no matter the answer is Yes or No.↵
↵
If the answer is Yes and we consider the outside characters of strings then DP of a smaller sub-trees is depending of DP of bigger sub-trees which is a contradiction. (Think if we use a button over the root of a sub-tree, then the sub-tree DP value changes)↵
↵
If the answer is No, then and we only consider key presses used on that sub-tree then there is another problem. Imagine we are solving a sub-tree $i$ with $j$ buttons and we assume we have solved all children of sub-tree rooted at $i$ root for all $j'\leq j$. If we use a button ending on $i$ we can't compute the total key presses of sub-tree rooted at $i$ because it depends on the amount of strings belonging to sub-trees rooted at $i$ children that are currently not being helped by any button, that do not depend on the sub-trees rooted at children of $i$ DP values. It is another variable that should be added as another DP variable. But I have not analysed if this way the problem could be solved, maybe is too hard.↵
↵
↵
Thus the conclusion is that one way or another we need to add another variable to characterize our DP sub problem, otherwise the sub-problems can't be connected properly.↵
↵
↵
We use the DP that answer YES to the question made above. We need to solve the fact stated about the dependency of smaller sub-trees with bigger sub-trees by adding a variable.↵
↵
DP would be defined like this↵
↵
$DP_{i,j,k}$: Amount of key presses to dial all strings belonging to sub-tree rooted at $i$ using $j$ buttons on them. (And k??)↵
↵
This of course depends of buttons used upside $i$. But there is a simplification. It depends in particular only on the lowest button used upside $i$, not every one.↵
↵
So that is the variable we need to make DP well defined, the lowest button used upside $i$. That is the only information about upper nodes that our sub-tree uses. ↵
↵
$DP_{i,j,k}$: Amount of key presses to dial all strings belonging to sub-tree rooted at $i$ using $j$ buttons on them, considering the lowest button used up-side $i$ ends at node $k$↵
↵
And now when we solve highest sub-trees using lower sub-trees, they don't depend on highest sub-trees, because when we solve a higher sub-tree using a lower sub-tree we know $k$ value that would correspond to the children DP solution we are using.↵
↵
[submission:48045680]↵
↵
↵
↵
↵
↵
↵
↵
↵
↵
↵
Hi. I will explain my approach to solve this hard and counter-intuitive problem.↵
↵
↵
**The problem**↵
------------------↵
↵
You've got some telephone numbers. Each number is dialed a given amount of times.↵
You would need to do an amount of key presses to dial all numbers, that is well defined.↵
However there is an amount of special buttons that can be used to type several digits at once. The buttons can only be used at the start of a number (before manually typing anything). Buttons presses does not count as digit press. Buttons can only be used once per number.↵
You need to decide how to place $k$ buttons to minimize total digit presses.↵
↵
↵
**
↵
We need to build a trie structure of the numbers, this will allow us to exploit numbers that have something in common↵
↵
![ ](
↵
**Need of DP**↵
↵
We should note some kind of DP should be used using sub-trees. The sub-problem could be defined like↵
↵
"Solving sub-tree rooted at $i$ using $j$ buttons on strings belonging to that sub-tree.↵
↵
But there is a technical problem defining DP like that. Although it is logically valid to think "solving" as "assigning buttons" to strings on that sub-tree, we need to store some number to represent that way of "assigning buttons" on DP_{ij}.↵
The natural value would be "total key presses" to solve the sub-tree $i$ using $j$ buttons. But think about the characters of strings belonging to sub-tree but that are outside it. ↵
Should we consider it on sub-problem value? Yes or No?↵
↵
I have though about that fact some time and realized that we can't solve the problem no matter the answer is Yes or No.↵
↵
If the answer is Yes and we consider the outside characters of strings then DP of a smaller sub-trees is depending of DP of bigger sub-trees which is a contradiction. (Think if we use a button over the root of a sub-tree, then the sub-tree DP value changes)↵
↵
If the answer is No, then and we only consider key presses used on that sub-tree then there is another problem. Imagine we are solving a sub-tree $i$ with $j$ buttons and we assume we have solved all children of sub-tree rooted at $i$ root for all $j'\leq j$. If we use a button ending on $i$ we can't compute the total key presses of sub-tree rooted at $i$ because it depends on the amount of strings belonging to sub-trees rooted at $i$ children that are currently not being helped by any button, that do not depend on the sub-trees rooted at children of $i$ DP values. It is another variable that should be added as another DP variable. But I have not analysed if this way the problem could be solved, maybe is too hard.↵
↵
↵
Thus the conclusion is that one way or another we need to add another variable to characterize our DP sub problem, otherwise the sub-problems can't be connected properly.↵
↵
↵
We use the DP that answer YES to the question made above. We need to solve the fact stated about the dependency of smaller sub-trees with bigger sub-trees by adding a variable.↵
↵
DP would be defined like this↵
↵
$DP_{i,j,k}$: Amount of key presses to dial all strings belonging to sub-tree rooted at $i$ using $j$ buttons on them. (And k??)↵
↵
This of course depends of buttons used upside $i$. But there is a simplification. It depends in particular only on the lowest button used upside $i$, not every one.↵
↵
So that is the variable we need to make DP well defined, the lowest button used upside $i$. That is the only information about upper nodes that our sub-tree uses. ↵
↵
$DP_{i,j,k}$: Amount of key presses to dial all strings belonging to sub-tree rooted at $i$ using $j$ buttons on them, considering the lowest button used up-side $i$ ends at node $k$↵
↵
And now when we solve highest sub-trees using lower sub-trees, they don't depend on highest sub-trees, because when we solve a higher sub-tree using a lower sub-tree we know $k$ value that would correspond to the children DP solution we are using.↵
↵
[submission:48045680]↵
↵
↵
↵
↵
↵
↵
↵
↵
↵
