CodeForces 237C

二分查找的代码还是写不出来，只能套用模板，本题只要找到区间内的素数然后用二分把最小的l找出来

#include<iostream>
#include<cstring>
#include<algorithm>
using namespace std;
int a,b,c;
const int maxn = 1000010;
int sum[maxn],pri[maxn];

 void init()
 {
     for(int i = 2;i < maxn;i++)
     {
        sum[i] = sum[i-1];
        if(pri[i])  continue;
            sum[i]++;
        for(int j = i+i;j < maxn;j += i)
            pri[j] = 1;
    }
}

bool check(int mid)
{
    for(int i = a;i <= b-mid+1;i++){
        if(sum[i+mid-1]-sum[i-1] < c)
        return 0;
    }
     return 1;
 }

int main()
{
    cin >> a>> b>> c;
    init();
    int x=0,y=0;
    if(sum[b]- sum[a-1] < c){
        cout<< "-1" << endl;
        return 0;
    }
    int l = 1,r = b-a+1,ans;
    while(l <= r)
    {
        int mid = (l+r)>>1;
        if(check(mid))  ans = mid,r = mid-1;
        else
            l= mid+1;
    }
    cout<< ans<< endl;

    return 0;
}