p087.java

/* 
 * Solution to Project Euler problem 87
 * Copyright (c) Project Nayuki. All rights reserved.
 * 
 * https://www.nayuki.io/page/project-euler-solutions
 * https://github.com/nayuki/Project-Euler-solutions
 */

import java.util.HashSet;
import java.util.Set;


public final class p087 implements EulerSolution {
	
	public static void main(String[] args) {
		System.out.println(new p087().run());
	}
	
	
	private static final int LIMIT = 50000000;
	
	public String run() {
		int[] primes = Library.listPrimes(Library.sqrt(LIMIT));
		
		Set<Integer> sums = new HashSet<>();
		sums.add(0);
		for (int i = 2; i <= 4; i++) {
			Set<Integer> newsums = new HashSet<>();
			for (int p : primes) {
				long q = 1;
				for (int j = 0; j < i; j++)
					q *= p;
				// q = p^i
				if (q > LIMIT)
					break;
				
				int r = (int)q;
				for (int x : sums) {
					if (x + r <= LIMIT)
						newsums.add(x + r);
				}
			}
			sums = newsums;
		}
		
		return Integer.toString(sums.size());
	}
	
}