# SPOJ-COINS

Problem: Given, a gold coin in Byteland, find the maximum amount of American dollars you can get for it.

EXPLANATION :

Consider a golden coin with a number ‘n’ on it. Now, the coin can be exchanged by n/2, n/3, n/4 coins only (all rounded down). Obviously, one would exchange the coins only if n/2+n/3+n/4 is greater than n. And similar holds for n/2, n/3, and n/4 and so on. Looking into the pattern carefully, all the numbers less than 12, if exchanged with coins, would return either lesser or equal to the number. So, it is better to exchange them directly by American dollars (1:1). Let’s try to solve it with a recursive function (a function in which we call our same function to solve it) :

Pseudocode of the function :

Return type coin (variable type n)
if(n<12)
return n;
else
return summation over i of (maximum of n/i and coin(n/i))
//here i is 2,3 and 4.


Ok, now an example, say 40: If I would exchange 40 by coins:-40/2+40/3+40/4=20+13+10=43. But that’s not the correct answer. 20=20/2+20/3+20/4=10+6+5=21 Now, as all the numbers have been reduced to numbers less than 12, they would return the same value. 13=13/2+13/3+13/4=6+4+3=13 10 being lesser than 12 would return the same value. So, 21+13+10= (44) American dollars would be the maximum one can get.

Considering 81: 81/2+81/3+81/4 = 40+27+20 40=20+13+10; 27=13+9+6=28; 20=10+6+5=21; Correct answer being 93.

Notice that 20 is being tested again (resulting from 40). We thus try to avoid solving a problem (which we have already solved once) again and again. For this we use dynamic programming (or memoization) which is the technique of storing already-calculated values for a fuction in an array (or vector or some other way) so that we can read those values directly instead of having to solve them again and again.

So, the pseudo code of the function finally becomes:

//Here we have used maps. One can use arrays or vectors as well.

static map<variable(n) type, function return type> memo;
if(n<12)
{return n;}
if(memo.count(n)>0)
return memo[n];
else
long long int ret;
ret= summation over i of (maximum of n/i and coin(n/i));  i=[2,3,4]
memo[n]=ret;
return ret;


This makes our solution pass well within the time-limit.