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

 1 #include<stdio.h>
 2 #include<string.h>
 3 int vis[30000005],sum[30000005];
 4 int main()
 5 {
 6   int T,i,j,t;
 7   memset(vis,0,sizeof(vis));
 8   for (i=1;i<=15000000;i++)
 9     for (j=2*i;j<=30000000;j+=i)
10       if ((j^(j-i))==i) vis[j]++;
11   sum[0]=0;
12   for (i=1;i<=30000000;i++)
13     sum[i]=sum[i-1]+vis[i];
14   scanf("%d",&T);
15   for (t=1;t<=T;t++)
16   {
17     scanf("%d",&i);
18     printf("Case %d: %d\n",t,sum[i]);
19   }
20 }

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