HDU 2136 找出某数的最大素数因子

Largest prime factor

Time Limit: 5000/1000 MS (Java/Others)    Memory Limit: 32768/32768 K (Java/Others)
Total Submission(s): 4524    Accepted Submission(s): 1595

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

1 2 3 4 5

 

 

Sample Output

0 1 2 1 3

 

思路：此题暴力会超时，网上看到的一个快速的求法，记录一下。

 

View Code

 1 #include <iostream>
 2 #include <stdio.h>
 3 #include <cstring>
 4 
 5 using namespace std;
 6 
 7 int prime[1000001];
 8 
 9 int main()
10 {
11     for(int i=2; i<=1000000; i++)
12         prime[i] = -1;
13     int num = 0;
14     for(int i=2; i<=1000000; i++)
15     {
16         if(prime[i] == -1)
17         {
18             num ++;
19             for(int j=i; j<=1000000; j+=i)
20                 prime[j] = num;
21         }
22     }
23     int n;
24     while(~scanf("%d",&n))
25     {
26         printf("%d\n",prime[n]);
27     }
28     return 0;
29 }