GCD 6種寫法

int GCD(int a, int b) {
        int d = 1;
        for (int i = 2; i <= std::min(a, b); ++i)  {
            if (0 == a % i && 0 == b % i) {
                d = i;
            }
        }
        return d;
    }

int GCD(int a, int b) {
        int d = min(a, b);  
        for (; d > 1; --d) {
            if (0 == a % d && 0 == b % d) {
                return d;
            }
        }
        return 1;
    }

輾轉消除法

int GCD(int a, int b) {
        if (a < b) {
            return GCD(b, a);
        }
 
        int r = a % b;
        if (r == 0) {
            return b;
        } else {
            return GCD(b, r);
        }
    }

int GCD(int a, int b) {
        if (a % b == 0) {
            return b;
        } else {
            return GCD(b, a % b);
        }
    }

int GCD(int a, int b) {
        return (b == 0) ? a : GCD(b, a % b);
    }

非遞迴版本

 int GCD(int a, int b) {
        while (b > 0) {
            int r = a % b;
            a = b;
            b = r;
        }
        return a;
    }

Reference
https://coherence0815.wordpress.com/2015/05/02/greatest-common-divisor-gcd-in-c/