Question

输出所有满足以下条件的三位整数:该数是素数,该数的个位数字十位数字之和被10除余数是该数的百位数字.

Answer 1

满足条件的数abc其实非常少,最多可能只有50个,去掉合数,只有十个左右.b + c - 10 = a或b+c=a

c = 1, 3, 5, 7, 9b = 0-9可以算出a小于33的质数,2,3,5,7,11,13,17,19,23,29,31

int isprime(int x) {

int i;

int const t[10] = {3,5,7,11,13,17,19,23,29,31};

for (i = 0; i < 10; ++i) {

if (x % t[i] == 0) return 0;

}

return 1;

}

int main() {

int a, b, c, x;

for ( b = 0; b < 10; ++b) {

for ( c = 1; c < 10; c += 2) {

a = (b + c < 10) ? b + c : b + c - 10;

x = a * 100 + b * 10 + c;

if (a > 0 isprime(x)) {

printf("%d ", x);

}

}

}

}