1. 质因数分解

public static List<Integer> factorize(int n){
        List<Integer>  factors = new ArrayList();
        int i;
        boolean isPrime = true;
        for (i = 2; i <= Math.sqrt(n); i++){
            if (n % i == 0){
                factors.add(i);
                isPrime = false;
                break;
            }
        }
        if (isPrime){
            factors.add(n);
        }else{
            factors.addAll(factorize( n / i));
        }
        return factors;
    }