hdu-2136-Largest prime factor [素数筛]

题目链接：http://acm.split.hdu.edu.cn/showproblem.php?pid=2136

Largest prime factor

Problem Description

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.

 

Input

Each line will contain one integer n(0 < n < 1000000).

 

Output

Output the LPF(n).

 

Sample Input

12345

 

Sample Output

01213

 

题目分析：设1的序号为0，2的序号为1，3的序号为2，5的序号为3，以此类推..要求输出所给定的数n的最大质因子在素数表的位置，0<n<1000000。 用素数筛进行打表就可以了。

#include<iostream>#include<cstring>using namespace std;const int maxx=1000001;int prim[maxx];int main(){    int i,j,n;    memset(prim,0,sizeof(prim));int cnt=1;for(i=2;i<maxx;i++){if(prim[i]==0){prim[i]=cnt;for(j=i*2;j<maxx;j+=i){prim[j]=cnt;}cnt++;}}while(~scanf("%d",&n))    {cout<<prim[n]<<endl;}    return 0;}

#include<iostream>#include<cstring>using namespace std;const int maxx=1000001;int prim[maxx];int main(){    int i,j,n;    memset(prim,0,sizeof(prim));int cnt=1;for(i=2;i<maxx;i++){if(prim[i]==0){for(j=i;j<maxx;j+=i){prim[j]=cnt;}cnt++;}}while(~scanf("%d",&n))    {cout<<prim[n]<<endl;}    return 0;}