GCD XOR, ACM/ICPC Dhaka 2013, UVa12716

不同的枚举方法，效率完全不同。值得记录一下！

 #include <cstdio>
 #include <cstring>
 int t, a, b, c, n, cas = , count = ;
 int cnt[];
 void pre() {
     count = ;
     memset(cnt, , sizeof(cnt));

     for (a = ; a <= ; a++) {
         for (c = ; c < a; c++) {
             if (a%c ==  && ((a-c)^a)==c) {
                 count++;
             }
         }
         cnt[a-] = count;
     }

 }
 int main(void)
 {

     for (scanf("%d", &t); t--;) {
         scanf("%d", &n);
         printf("Case %d: %d\n", ++cas, cnt[n-]);
     }
     return ;
 }

 #include <cstdio>
 #include <cstring>
 int t, a, b, c, n, cas = , count = ;
 const int maxn = ;
 int cnt[maxn];
 void pre() {
     count = ;
     memset(cnt, , sizeof(cnt));

     for (c = ; c <= maxn/; c++) {
         for (a = c+c; a <= maxn; a+=c) {
             if (((a-c)^a)==c) {
                 cnt[a]++;
             }
         }
     }
     for (int i = ; i <= maxn; i++) {
         cnt[i] += cnt[i-];
     }

 }
 int main(void)
 {
     pre();
     for (scanf("%d", &t); t--;) {
         scanf("%d", &n);
         printf("Case %d: %d\n", ++cas, cnt[n]);
     }
     return ;
 }