uva12716 n以内有多少对整数a、b满足(1≤b≤a)且gcd(a, b) = xor(a, b)

额现在才补==

这题当时是cp各种姿势打表发现的==然后我大胆用了类似筛法写了一下，竟然给过了==

规律：如果gcd(a, b) = xor(a, b) = c， 则 c = a - b

然后就能利用c来筛了

#include<stdio.h>
#include<string.h>
int vis[30000005],sum[30000005];
int main()
{
  int T,i,j,t;
  memset(vis,0,sizeof(vis));
  for (i=1;i<=15000000;i++)
    for (j=2*i;j<=30000000;j+=i)
      if ((j^(j-i))==i) vis[j]++;
  sum[0]=0;
  for (i=1;i<=30000000;i++)
    sum[i]=sum[i-1]+vis[i];
  scanf("%d",&T);
  for (t=1;t<=T;t++)
  {
    scanf("%d",&i);
    printf("Case %d: %d\n",t,sum[i]);
  }
}

题目链接：http://acm.hust.edu.cn/vjudge/problem/viewProblem.action?id=49096