/* * Solution to Project Euler problem 75 * 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 p075 implements EulerSolution { public static void main(String[] args) { System.out.println(new p075().run()); } private static final int LIMIT = 1500000; public String run() { /* * Pythagorean triples theorem: * Every primitive Pythagorean triple with a odd and b even can be expressed as * a = st, b = (s^2-t^2)/2, c = (s^2+t^2)/2, where s > t > 0 are coprime odd integers. */ Set triples = new HashSet<>(); for (int s = 3; s * s <= LIMIT; s += 2) { for (int t = s - 2; t > 0; t -= 2) { if (Library.gcd(s, t) == 1) { int a = s * t; int b = (s * s - t * t) / 2; int c = (s * s + t * t) / 2; if (a + b + c <= LIMIT) triples.add(new IntTriple(a, b, c)); } } } byte[] ways = new byte[LIMIT + 1]; for (IntTriple triple : triples) { int sum = triple.a + triple.b + triple.c; for (int i = sum; i < ways.length; i += sum) ways[i] = (byte)Math.min(ways[i] + 1, 2); // Increment but saturate at 2 } int count = 0; for (int x : ways) { if (x == 1) count++; } return Integer.toString(count); } private static final class IntTriple { public final int a; public final int b; public final int c; public IntTriple(int a, int b, int c) { this.a = a; this.b = b; this.c = c; } public boolean equals(Object obj) { if (!(obj instanceof IntTriple)) return false; else { IntTriple other = (IntTriple)obj; return a == other.a && b == other.b && c == other.c; } } public int hashCode() { return a + b + c; } } }