Largest prime factor (素数筛）

Everybody knows any number can be combined by the prime number.
Now, your task is telling me what position of the largest prime factor.
The position of prime 2 is 1, prime 3 is 2, and prime 5 is 3, etc.
Specially, LPF(1) = 0.

 

筛出来放在表里直接输出

#include <cstdio>
const int MAX=1000000;
int prime[MAX]={0};
int mark[MAX]={0};

void init()
{
    for(int i=2,n=1;i<MAX;i++) 
        {
            if(prime[i]==0)        
                {
                    mark[i]=n++;  
                    for(int j=i;j<MAX;j=j+i)    
                        {
                            prime[j]=i;   //筛掉i的倍数         
                        }
                }
        }
}
int main()
{
    int n;
    init();
    while(~scanf("%d",&n))
        {
            printf("%d\n",mark[prime[n]]);    //prime中存放这个数的最大质因数 
        }
    return 0;
}