2520是最小的能被1-10中每個數字整除的正整數。

最小的能被1-20中每個數整除的正整數是多少？

分解質因數，不多說

public class Problem5 {

    private static HashSet<Integer> set = new HashSet<Integer>();
public static void main(String[] args) {

        for (int i = 1; i <= 20; i++) {
            getPrime(i);
}

        Iterator<Integer> it = set
 
.iterator();
int res = 1;
while (it.hasNext()) {
            int prime = it.next();
res = prime * res;
}

        System.out.println(res);
}

    private static void getPrime(int value) {

        if (isPrime(value)) {
            set.add(value);
} else {
            for (int i = 2; i < value / 2; 
 
i++) {

                int remainder = value % i;
if (remainder == 0 || isPrime(i)) {
                    set.add(i);
}
            }
        }

    }


    public static Boolean isPrime(int value) {
        Boolean flag = true;
for (int i = 2; i <= Math.sqrt(value); i++) {
            if (value % i == 0
 
) {
                flag = false;
break;
}
        }
        return flag;
}
}