HDU 2136 Largest prime factor【素数筛选法】

题意：给你一个数，叫你求这个数的最大的素数因子在素数表中的位置。

Largest prime factor

Time Limit: 5000/1000 MS (Java/Others)    

Memory Limit: 32768/32768 K (Java/Others)

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

 

Author

Wiskey

 

Source

HDU 2007-11 Programming Contest_WarmUp

#include<stdio.h>#include<string.h>#define MAXN 1000000+1000int prime[MAXN];int main(){int k=1;//初始化k的位置为1int i,j,n;memset(prime,0,sizeof(prime));

/*筛选的位置*/for(i=2;i<MAXN;i++)//要到MAXN，而不是sqrt(MAXN){if(!prime[i])//if(prime[i]==0){prime[i]=k;//素数i的位置for(j=i+i;j<MAXN;j+=i){prime[j]=k;//构造出j的暂时最大素数因子的位置}k++;}}while(~scanf("%d",&n)){printf("%d\n",prime[n]);}return 0;}