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p023.java
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p023.java
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/*
* Solution to Project Euler problem 23
* By Nayuki Minase
*
* http://nayuki.eigenstate.org/page/project-euler-solutions
* https://github.com/nayuki/Project-Euler-solutions
*/
public final class p023 implements EulerSolution {
public static void main(String[] args) {
System.out.println(new p023().run());
}
private boolean[] isAbundant = new boolean[28124];
public String run() {
// Compute look-up table
for (int i = 1; i < isAbundant.length; i++)
isAbundant[i] = isAbundant(i);
int sum = 0;
for (int i = 1; i <= 28123; i++) {
if (!isSumOf2Abundants(i))
sum += i;
}
return Integer.toString(sum);
}
private boolean isSumOf2Abundants(int n) {
for (int i = 0; i <= n; i++) {
if (isAbundant[i] && isAbundant[n - i])
return true;
}
return false;
}
private static boolean isAbundant(int n) {
if (n < 1)
throw new IllegalArgumentException();
int sum = 1; // Sum of factors
int end = Library.sqrt(n);
for (int i = 2; i <= end; i++) {
if (n % i == 0)
sum += i + n / i;
}
if (end * end == n)
sum -= end;
return sum > n;
}
}