XDOJ_1028_素数

#include<iostream>
#include<cstring>
#include<cstdio>
#include<algorithm>
using namespace std;

int dp[1000005] = {0},prime[100000];
int main()
{
    int cnt = 0;
    for(int i = 2;i <= 1000000;i++)
    {
        if(dp[i])    continue;
        for(int j = 2*i;j <= 1000000;j += i)    dp[j] = 1;
    }
    for(int i = 2;i <= 1000000;i++)
    {
        if(!dp[i])    prime[cnt++] = i;
    }
    memset(dp,0x3f,sizeof(dp));
    dp[1] = 0;
    for(int i = 1;i <= 1000000;i++)
    {
        dp[i+1] = min(dp[i]+1,dp[i+1]);
        for(int j = 0;j < cnt;j++)
        {
            long long k = (long long)i*prime[j];
            if(k > 1000000)    break;
            dp[k] = min(dp[i]+1,dp[k]);
        }
    }
    int n;
    while(~scanf("%d",&n))  printf("%d\n",dp[n]);
    return 0;
}