Codeforces Round #147 (Div. 2) C. Primes on Interval 二分

C. Primes on Interval

链接：

http://codeforces.com/contest/237/problem/C

代码：

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <functional>
using namespace std;
#define rep(i,a,n) for (int i=a;i<=n;i++)
#define per(i,a,n) for (int i=n;i>=a;i--)
#define pb push_back
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define SZ(x) ((int)(x).size())
typedef vector<int> VI;
typedef long long ll;
typedef pair<int, int> PII;
const ll mod = 1000000007;
// head

const int maxn = 1e6 + 7;
int prime[maxn];
int a, b, k;

void IsPrime() {
    prime[0] = prime[1] = 0; prime[2] = 1;
    for (int i = 3; i<maxn; i++)
        prime[i] = i % 2 == 0 ? 0 : 1;
    int t = (int)sqrt(maxn*1.0);
    for (int i = 3; i <= t; i++)
        if (prime[i])
            for (int j = i*i; j<maxn; j += 2 * i)
                prime[j] = 0;
}

bool check(int m) {
    rep(i, a, b - m + 1) if (prime[i + m - 1] - prime[i - 1] < k) return false;
    return true;
}

int main()
{
    IsPrime();
    cin >> a >> b >> k;
    rep(i, a, b) prime[i] = prime[i - 1] + prime[i];
    prime[a - 1] = 0;
    int l = 1, r = b - a + 1, ans = maxn;
    while (l <= r) {
        int mid = (l + r) / 2;
        if (check(mid)) {
            r = mid - 1;
            ans = mid;
        }
        else l = mid + 1;
    }
    if (ans == maxn) cout << -1 << endl;
    else cout << ans << endl;
    return 0;
}