Irrelevant Elements, ACM/ICPC NEERC 2004, UVa1635

这种题目最重要的是思路了清晰

 #include <cstdio>
 #include <cstring>
 const int maxn = ;//sqrt(n)+1 is enough

 int fac[][];
 int fac_c[];
 int a[maxn];
 void factor(int m) {
     int& num = fac[][];
     num = ;
     for (int i = ; i*i <= m; i++) {
         if(m%i == ) {
             fac[++num][] = i;
             fac[num][] = ;
             do {
                 fac[num][]++;
                 m/= i;
             } while(m%i == );
         }
     }
     if (m > ) {
         fac[++num][] = m;
         fac[num][] = ;
     }
 }
 bool check(int n, int j) {
     int num = fac[][];
     int a = n - j;
     int b = j;
     for (int i = ; i <= num; i++) {
         //为了提高效率，只用检验m的分解式中的素因数
         int p = fac[i][];
         int& q = fac_c[i];
         for (; a%p == ; a/=p, q++);
         for (; b%p == ; b/=p, q--);
     }
     for (int i = ; i <= num; i++) {
         if (fac[i][] > fac_c[i]) {
             return false;
         }
     }
     return true;
 }
 int main(void) {
     int n, m;
     for (; scanf("%d%d", &n, &m)==;printf("\n")) {
         int cnt = ;
         factor(m);
         memset(fac_c, , sizeof(fac_c));
         for (int i = ; i < n; i++) {
             if (check(n, i)) {
                 a[cnt++] = i+;
             }
         }
         printf("%d\n", cnt);
         for (int i = ; i < cnt; i++) {
             printf("%s%d",i==?"":" ", a[i]);
         }
     }
     return ;
 }