common divisors两个数字的所有共同除数

思路：就是把gcd拆一下，因为如果GCD可以，这些数就也可以
3，6

gcd = n = 3

1/3 没了，0 += 2

GCD辗转相除法：b一开始是除数，后来是余数。余数为0就行了

16 4

4，4

4，0

所以返回a = 4

https://www.geeksforgeeks.org/common-divisors-of-two-numbers/

// Java implementation of program 

class Test { 
    // method to calculate gcd of two numbers 
    static int gcd(int a, int b) 
    { 
        if (a == 0) 
            return b; 

        return gcd(b % a, a); 
    } 
    // method to calculate all common divisors 
    // of two given numbers 
    // a, b --> input integer numbers 
    static int commDiv(int a, int b) 
    { 
        // find gcd of a, b 
        int n = gcd(a, b); 

        // Count divisors of n. 
        int result = 0; 
        for (int i = 1; i <= Math.sqrt(n); i++) { 
            // if 'i' is factor of n 
            if (n % i == 0) { 
                // check if divisors are equal 
                if (n / i == i) 
                    result += 1; 
                else
                    result += 2; 
            } 
        } 
        return result; 
    } 

    // Driver method 
    public static void main(String args[]) 
    { 
        int a = 12, b = 24; 
        System.out.println(commDiv(a, b)); 
    } 
}