求第N个素数

题意:

　　输入一个数字N(N <= 200000),输出底N个素数.

思路:

　　1-3000000中大约有210000素数.直接把这些素数打表,然后离线查询就OK.

代码:

#include <iostream>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <algorithm>
#include <queue>
#include <stack>
#include <vector>
using namespace std;
typedef long long LL;

bool visit[3010000 + 7];  
int prime[3000000 + 7];  
int ans[220000 + 7];
void init_prim(){  
    memset(visit, true, sizeof(visit));  
    int num = 0;  
    for (int i = 2; i <= 3000000; ++i){  
        if (visit[i] == true){  
            num++;  
            prime[num] = i;  
        }  
        for (int j = 1; ((j <= num) && (i * prime[j] <= 3000000));  ++j){  
            visit[i * prime[j]] = false;  
            if (i % prime[j] == 0) break; 
        }  
    }  
}

int main()
{
    //freopen("input.txt", "r", stdin);
    memset(prime, 0, sizeof(prime));
    init_prim();
    int cnt = 0;
    for(int i = 0; i <= 3000000; i++){
        if(prime[i])ans[cnt++] = prime[i];
    }
    int t;
    scanf("%d", &t);
    while(t--){
        int n;
        scanf("%d", &n);
        printf("%d
", ans[n - 1]);
    }
    return 0;
}