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pe3.java
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pe3.java
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package euler;
public class pe3 {
/*The prime factors of 13195 are 5, 7, 13 and 29.
What is the largest prime factor of the number 600851475143 ?*/
public static void main(String[] args) {
System.out.println(getAnswer());
}
public static boolean[] sieve;
public static int getAnswer() {
int largestFactor = 0; //largestFactor starts at 0
sieve = new boolean[(int)Math.sqrt(600851475143L) + 1]; //Primes sieve of boolean values up to sqrt of target
for (int i = 0; i < sieve.length; i++) { //Fill sieve with true values
sieve[i] = true;
}
sieve[0] = false; //0 and 1 aren't prime
sieve[1] = false;
for (int i = 0; i < sieve.length; i++) { //Find first true value. It will be a prime
if (sieve[i]) { //If it is a factor of target, it is new largest factor
if (600851475143L % i == 0) {
largestFactor = i;
}
foundPrime(i); //Remove all multiples of found prime from sieve
}
}
return largestFactor;
}
public static void foundPrime(int n) { //Removes all multiples of a prime number from sieve
for (int i = n * 2; i < sieve.length; i += n) {
sieve[i] = false;
}
}
}