题目链接：http://www.spoj.com/problems/FRQPRIME/
题目大意：询问[2,N]区间内的包含素数个数至少为K的区间的个数。
解题思路：素数个数前缀和+二分。哪儿位大侠有好方法的话希望在评论区说明。
代码：
1 const int inf = 0x3f3f3f3f; 2 const int maxn = 1e6 + 5; 3 int sum[maxn]; 4 bool vis[maxn]; 5 int n, k; 6 7 void dowork(){ 8 memset(vis, 0, sizeof(vis)); 9 memset(sum, 0, sizeof(sum)); 10 for(int i = 2; i < maxn; i++) 11 for(int j = i + i; j > 0 && j < maxn; j += i) 12 vis[j] = 1; 13 for(int i = 2; i < maxn; i++) 14 if(!vis[i]) sum[i] = sum[i - 1] + 1; 15 else sum[i] = sum[i - 1]; 16 17 } 18 void solve(){ 19 if(k == 0){ 20 ll u = n; 21 ll ans = u * (u - 1) / 2; 22 printf("%lld ", ans); 23 return; 24 } 25 ll ans = 0; 26 for(int i = 2; i <= n; i++){ 27 int u = sum[i - 1] + k; 28 int v = lower_bound(sum, sum + n + 1, u) - sum; 29 ll x = n - v + 1; 30 ans += x; 31 } 32 printf("%lld ", ans); 33 } 34 int main(){ 35 dowork(); 36 int t; 37 scanf("%d", &t); 38 while(t--){ 39 scanf("%d %d", &n, &k); 40 solve(); 41 } 42 }
题目：
FRQPRIME - Frequent Prime Ranges
A range [L..H] is called a K-Frequent Prime range if there are atleast K primes amongst the numbers L,L+1,..,H. Given N and K, calculate how many subranges of the range [2..N] are K-Frequent Prime.
Input
The first line contains the number of test cases T. Each of the next T lines contains 2 integers N and K.
Output
Output T lines, one corresponding to each test case, containing the required answer.
Example
Sample Input :
4
2 1
5 2
5 1
9 3
Sample Output :
1
4
9
8
Note : For the first test case, the only valid subrange is [2..2], whereas for the second test case, the valid subranges are : [2..3],[2..4],[2..5],[3..5].
Constraints
1 <= T <= 100
2 <= N <= 100000
0 <= K <= 10000