Problem:
There are some prime values, p, for which there exists a positive integer, n, such that the expression n3 + n2p is a perfect cube.
For example, when p = 19, 83 + 82[×]19 = 123.
What is perhaps most surprising is that for each prime with this property the value of n is unique, and there are only four such primes below one-hundred.
How many primes below one million have this remarkable property?
For example, when p = 19, 83 + 82[×]19 = 123.
What is perhaps most surprising is that for each prime with this property the value of n is unique, and there are only four such primes below one-hundred.
How many primes below one million have this remarkable property?
Solution:
73682
Code:
The solution may include methods that will be found here: Library.java .
public interface EulerSolution{
public String run();
}
/*
* Solution to Project Euler problem 31
* By Nayuki Minase
*
* http://nayuki.eigenstate.org/page/project-euler-solutions
* https://github.com/nayuki/Project-Euler-solutions
*/
public final class p031 implements EulerSolution {
public static void main(String[] args) {
System.out.println(new p031().run());
}
private static final int TOTAL = 200;
private static int[] COINS = {1, 2, 5, 10, 20, 50, 100, 200};
public String run() {
// Knapsack problem. ways[i][j] is the number of ways to use
// any of the first i coin values to form an unordered sum of j.
int[][] ways = new int[COINS.length + 1][TOTAL + 1];
ways[0][0] = 1;
for (int i = 0; i < COINS.length; i++) {
for (int j = 0; j <= TOTAL; j++)
ways[i + 1][j] = ways[i][j] + (j >= COINS[i] ? ways[i + 1][j - COINS[i]] : 0); // Dynamic programming
}
return Integer.toString(ways[COINS.length][TOTAL]);
}
}
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