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Topic: hard coin problem (Read 3693 times) |
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RajivG
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hard coin problem
« on: Aug 4th, 2011, 4:12am » |
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we have n coins in a row with different denomination on it and placed randomly
game rule says that either left most or right most coin can be picked and only two players can play this game (one will pick one coin at a time)
now we have to come up with a sequence which make me winner (my sum of denomination of n/2 coins should greater than sum of denomination of remaining n/2 coins)
I tried to solve this but every time I ended up reducing this problem to NP
greedy approach will not work i deduced that but i am not able to find DP solution as well
but interviewer said there is a solution for it because we restrict on possible sequences
any help
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Grimbal
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Re: hard coin problem
« Reply #1 on: Aug 4th, 2011, 6:17am » |
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The way I understand the problem, you just need to evaluate the best sum for each contiguous sequence of coins. And there are O(n^2) of them.
If faced with an even number of coins you can guarantee to get half the available value. Compute the sum of the coins at even and the sum of the coins at odd positions. If for instance the set of evens are worth more, always take the coin from the even set, leaving the other player with two coins from the odd set to choose from.
I don't know if it is an optimal strategy, though.
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paras
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Re: hard coin problem
« Reply #2 on: Aug 4th, 2011, 12:06pm » |
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it is given that number of coins is even.
Calculate sum of coins placed at even position and sum of coins placed at odd position.
make the other person pick only smaller sum position coins by picking the bigger sum position coins 
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inexorable
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Re: hard coin problem
« Reply #3 on: Aug 4th, 2011, 12:06pm » |
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You could do by dynamic programming by using the gain/loss of player1 over player 2 for a sequence of n-1 coins to find the gain/loss for sequence of n coins.
here instead of using actual sums player1 or player2 get, using the gain/loss the first player gets for a given sequence of length n would make DP easy.
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agent_purge
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Re: hard coin problem
« Reply #4 on: Aug 6th, 2011, 10:04am » |
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Solved using DP in O(n2)
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Let coins be 1..N.
d[i,j] = gain (could be negative too) of player with current move when coins from i to j are remaining
d[i,j] = min{ c[i] - d[i-1,j],
c[j] - d[i,j-1] }
d[0,0] = 0
d[i,i] = c[i]
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towr
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Re: hard coin problem
« Reply #5 on: Aug 6th, 2011, 1:47pm » |
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on Aug 4th, 2011, 6:17am, Grimbal wrote:| I don't know if it is an optimal strategy, though. |
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It doesn't lead you to win in every game where you could win, for instance 3,2,1,2. Sticking with just the odd or even set means you draw, but if you switch after each of you had a turn, you can win.
So for drawn odds/evens, you can modify the strategy to switch once you're ahead. Though I don't know if that's optimal.
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sat90
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Re: hard coin problem
« Reply #6 on: Aug 7th, 2011, 1:19am » |
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compare the two coins at end and take decision
my code:
i=0;
j=n;
while(i<=j)
{
if(a[i]<=a[j]) //compare
{
me[k++]=j; //player1 move
j--;
if(a[i]<=a[j]) //player2 move
i++;
else
j--;
}
else
{
me[k++]=i;
i++;
if(a[i]<=a[j])
i++;
else
j--;
}
}
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sat90
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Re: hard coin problem
« Reply #7 on: Aug 7th, 2011, 1:20am » |
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time taken will be O(n).
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towr
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Re: hard coin problem
« Reply #8 on: Aug 7th, 2011, 9:47pm » |
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A greedy algorithm is not optimal for this problem. Just take 3,10,2,1 as example, you'd get 5 as first player and the second player would get 11.
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wiley
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Re: hard coin problem
« Reply #9 on: Aug 13th, 2011, 2:28am » |
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Winning player must have choice to be player1(1st turn) or player2(2nd turn) in game.
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towr
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Re: hard coin problem
« Reply #10 on: Aug 13th, 2011, 1:12pm » |
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Why would you say that? The second player can never win, since the first can always draw or do better; so there wouldn't be a reason to choose to be second player.
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wiley
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Re: hard coin problem
« Reply #11 on: Aug 17th, 2011, 11:31am » |
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on Aug 13th, 2011, 1:12pm, towr wrote:| Why would you say that? The second player can never win, since the first can always draw or do better; so there wouldn't be a reason to choose to be second player. |
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Yeah, you are right. Had got wrong picture in my head..
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Hippo
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Re: hard coin problem
« Reply #12 on: Sep 12th, 2011, 1:21pm » |
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on Aug 7th, 2011, 9:47pm, towr wrote:| A greedy algorithm is not optimal for this problem. Just take 3,10,2,1 as example, you'd get 5 as first player and the second player would get 11. |
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But for sequence like 2,5,25,6,1,16 the parity strategy gives 28-27 while there is possibility to gain 42-13.
Seems to me optimal strategy is to take difference of the border two numbers and chose the end with higher difference.
(16-1)>(2-5) ... take 16 (2-5)>(1-6) ... take 2
(1-6)>(5-25) ... take 1 (6-25)>(5-25) ... take 6
(25-5)>(5-25) ... take 25 ... hmm algorithm fails so take the last number 5.
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| « Last Edit: Sep 12th, 2011, 1:38pm by Hippo » |
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towr
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Re: hard coin problem
« Reply #13 on: Sep 12th, 2011, 10:04pm » |
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on Sep 12th, 2011, 1:21pm, Hippo wrote:| But for sequence like 2,5,25,6,1,16 the parity strategy gives 28-27 while there is possibility to gain 42-13. |
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Winning is winning. As long as you win when you can, I don't see how winning in a different way is more optimal.
Quote:| Seems to me optimal strategy is to take difference of the border two numbers and chose the end with higher difference. |
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Try that on 99,100,101,102,4,6,8,10
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| « Last Edit: Sep 12th, 2011, 10:10pm by towr » |
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Hippo
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Re: hard coin problem
« Reply #14 on: Sep 13th, 2011, 4:53am » |
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on Sep 12th, 2011, 10:04pm, towr wrote:
Winning is winning. As long as you win when you can, I don't see how winning in a different way is more optimal.
Try that on 99,100,101,102,4,6,8,10 |
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Nice refutation
OK, the question was to have positive difference.
Other question is to maximize the difference ...
OK suppose we have to gain just possitive difference and the odd sum equals even sum....
For odd player taking maximal border number is optimal counterstrategy as it gives \sumeven and \sumodd no border with one of border options.
Even player choses maximum so making \sumodd minimal gives better chances.
For sequence 100,103,25,32,20,8,30,33,46,38,8,15
Starting from left leads to difference 0, while starting from righ leads to difference 10 due to switch on 8,30,33,46,38,8 subsequence.
When starting from left the opponent switches on 8,30,33,46,38,8,15 what leads to the original even/odd partition.
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| « Last Edit: Sep 13th, 2011, 5:56am by Hippo » |
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RajivG
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Re: hard coin problem
« Reply #15 on: Oct 12th, 2011, 2:07am » |
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on Aug 4th, 2011, 4:12am, RajivG wrote:we have n coins in a row with different denomination on it and placed randomly
game rule says that either left most or right most coin can be picked and only two players can play this game (one will pick one coin at a time)
now we have to come up with a sequence which make me winner (my sum of denomination of n/2 coins should greater than sum of denomination of remaining n/2 coins)
I tried to solve this but every time I ended up reducing this problem to NP
greedy approach will not work i deduced that but i am not able to find DP solution as well
but interviewer said there is a solution for it because we restrict on possible sequences
any help
|
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finally i got a solution for it, (though it took so many days to come out 4m my mind)
posting here to get it reviewed if solution has any flaw
let us say sequence is
2,5,8,10,25,26,21,6,1,4
now start creating a simple binary tree (choice tree more correctly)
you have choice to pick either 2 or 4
........................0
...................2.......4
now further if we pick 2 then we have choice for picking 5 or 4 and if we pick 4 then we have choice for 2 and 1
so now tree looks like
........................0
.................2..........4
.............5.....4...2.......1
this way we will keep expanding our tree
once tree is done then we just need to find a path on which the sum is higher (which is simple to find)
so max sum path is our answer
let me know if i am wrong some where
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Hippo
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Re: hard coin problem
« Reply #16 on: Oct 12th, 2011, 6:32am » |
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on Oct 12th, 2011, 2:07am, RajivG wrote:
finally i got a solution for it, (though it took so many days to come out 4m my mind)
posting here to get it reviewed if solution has any flaw
let us say sequence is
2,5,8,10,25,26,21,6,1,4
now start creating a simple binary tree (choice tree more correctly)
you have choice to pick either 2 or 4
........................0
...................2.......4
now further if we pick 2 then we have choice for picking 5 or 4 and if we pick 4 then we have choice for 2 and 1
so now tree looks like
........................0
.................2..........4
.............5.....4...2.......1
this way we will keep expanding our tree
once tree is done then we just need to find a path on which the sum is higher (which is simple to find)
so max sum path is our answer
let me know if i am wrong some where
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It must be wrong ... you forgot there is another player ... so if you choose one parity and change sign in the depths of the chosen parity ... it would correspond to the game, but with minimax search ... .
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RajivG
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Re: hard coin problem
« Reply #17 on: Oct 12th, 2011, 7:21am » |
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on Oct 12th, 2011, 6:32am, Hippo wrote:
It must be wrong ... you forgot there is another player ... so if you choose one parity and change sign in the depths of the chosen parity ... it would correspond to the game, but with minimax search ... . |
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wait.... problem says that i need to give a winning sequence only, if not followed player may loose (loosing reason could be same as you said)
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Dufus
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Re: hard coin problem
« Reply #18 on: Dec 25th, 2011, 5:02am » |
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FWIW, I came across a similar problem,
Consider a row of n coins of values v(1) ... v(n), where n is even. We play a game against an opponent by alternating turns. In each turn, a player selects either the first or last coin from the row, removes it from the row permanently, and receives the value of the coin. Determine the maximum possible amount of money we can definitely win if we move first.
It seems the problem can be solved using Dynamic programming in O(N2) time and O(N2) extra space.
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birbal
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Re: hard coin problem
« Reply #19 on: Dec 26th, 2011, 9:22am » |
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on Dec 25th, 2011, 5:02am, Dufus wrote:FWIW, I came across a similar problem,
Consider a row of n coins of values v(1) ... v(n), where n is even. We play a game against an opponent by alternating turns. In each turn, a player selects either the first or last coin from the row, removes it from the row permanently, and receives the value of the coin. Determine the maximum possible amount of money we can definitely win if we move first.
It seems the problem can be solved using Dynamic programming in O(N2) time and O(N2) extra space. |
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Is it similar? It looks exactly the same problem to me 
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towr
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Re: hard coin problem
« Reply #20 on: Dec 26th, 2011, 10:06am » |
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In the original problem you only have to win, here you want to get the maximum score (which usually means you win unless you draw, but it's typically more effort as well).
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