倍数，艾氏筛法思想

题目链接：http://codeforces.com/problemset/problem/632/D

题目大意：给你n个数，找出最多的数使他们都能被x (x<=m) 整除

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cstdlib>
#include <cmath>
#include <set>
#include <map>
#include <vector>
using namespace std;

const int MAX = 1e6 + 10;
int res[MAX], a[MAX];
struct data{
    int x, y;
}num[MAX];
int main()
{
    int n, m, i, j, t = 1;
    scanf("%d%d", &n, &m);
    for(i = 0; i <= m; i++)
    {
        num[i].x = 0;
        num[i].y = 0;
    }
    for(i = 0; i < n; i++)
    {
        scanf("%d", a + i);
        if(a[i] <= m)
        {
            num[a[i]].x++;
            num[a[i]].y++;
        }
    }
    if(m > 1)
    {
        for(i = 2; i <= m; i++)
        {
            if(num[i].x > 0)
            { 
                if(num[i].y > num[t].y)
                        t = i;
                for(j = i * 2; j <= m; j += i)
                {
                    num[j].y += num[i].x;
                    if(num[j].y > num[t].y)
                        t = j;
                }
            }
        }
    }
    j = 0;
    for(i = 0; i < n; i++)
    {
        if(a[i] <= m && t % a[i] == 0)
            res[j++] = i;
    }    
    printf("%d %d\n", t, j);
    for(i = 0; i < j; i++)
    {
        if(i != j - 1)
            printf("%d ", res[i] + 1);
        else
            printf("%d", res[i] + 1);
    }
    printf("\n");
    return 0;
}