POJPrime Gap 素数筛选+二分查找

问你一个数在一个连续区间，这个区间全为和数的长度。

代码如下：

#include <cstring>
#include <cstdio>
#include <cstdlib>
#include <algorithm>
#define MAXN 1300000
using namespace std;

int p[MAXN+5], rec[100005], N;

void pre()
{
    int k;
    for (int i = 4; i <= MAXN; i += 2) {
        p[i] = 1;
    }    
    for (int i = 3; i <= 1141; i += 2) {
        if (!p[i]) {
            k = 2 * i;
            for (int j = i * i; j <= MAXN; j += k) {
                p[j] = 1;    
            }    
        }
    }
    for (int i = 2, j = 0; j <= 100000; ++i) {
        if (!p[i]) {
            rec[++j] = i;
        }
    }
}

int bsearch(int l, int r, int x, int f)
{
    int mid;
    while (l <= r) {
        mid = (l + r) >> 1;
        if (x < rec[mid]) {
            r = mid - 1;
        }
        else {
            l = mid + 1;
        }
    }    
    if (f == 1) {
        return rec[r];
    }
    else {
        return rec[l];
    }
}

int main()
{
    pre();
    int l, r;
    while (scanf("%d", &N), N) {
        if (!p[N]) {
            puts("0");
            continue;
        }
        l = bsearch(1, 100000, N, 1);
        r = bsearch(1, 100000, N, 2);
        printf("%d\n", r - l);
    }
    return 0;    
}