HDU 1216 Assistance Required 埃拉托色尼色筛法

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=1216

思路：色筛法

代码（1）：

#include<iostream>//--------1216 HDU   埃拉托色尼筛选法
#include<stdio.h>
#include<stdlib.h>
#include<algorithm>
#include<math.h>
#include<string.h>
#include<vector>
using namespace std;
#define Max 40000
bool b[Max];
__int64 a[Max], c;
void fun()
{
    __int64 i, j, count;
    c = 0;
    memset(b, true, sizeof(b));
    b[1] = 0;
    for (i = 2; i < Max; ++i)
    {
        if (b[i])
        {
            a[++c] = i;
            count = i;
            for (j = i+1; j < Max; ++j)
            {
                if (b[j] == true)
                    count--;
                if (count == 0)
                {
                    b[j] = false;
                    count = i;
                }
            }
        }
    }
}
int main()
{
    fun();
    int n;
    while (scanf("%d", &n) != EOF)
    {
        if (n == 0)
            break;
        else
            printf("%d\n", a[n]);
    }
    return 0;
}

基本类似的另外一种：

#include<iostream>
#include<stdio.h>
#include<cstdlib>
#include<string.h>
#include<algorithm>
#include<math.h>
using namespace std;
#define Max 40000
__int64 ans[Max];
__int64 pop[3001];
void dfs()
{
    __int64 i, j, count;
    for (i = 2; i < Max; i++)
    {
        ans[i] = i;
    }
    for (i = 2; i < Max; i++)
    {
        if (ans[i] != 0)
        {
            count = i;
            for (j = i + 1; j < Max; j++)
            {
                if (ans[j] != 0)
                    count--;
                if (count == 0)
                {
                    ans[j] = 0;
                    count = i;
                }
            }
        }
        
    }
    j = 2;
    for (i = 1; i <= 3000; i++)
    {
        while (ans[j] == 0)
        {
            j += 1;
        }
        if (ans[j] != 0)
        {
            pop[i] = ans[j];
            j += 1;
        }
    }
}
int main()
{
    dfs();
    int n;
    while (scanf("%d", &n)&&n)
    {
            printf("%d\n", pop[n]);
    }
    return 0;
}