HDU 2136 Largest prime factor

http://acm.hdu.edu.cn/showproblem.php?pid=2136

 

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

 

题解：素数筛 普通筛法

时间复杂度：$O(N * lnN)$

代码：

#include <bits/stdc++.h>
using namespace std;

const int maxn = 1e6 + 10;
int a[maxn];

int main() {
    int N, k = 1;
    memset(a, 0, sizeof(a));
    for(int i = 2; i < maxn; i ++) {
        if(a[i] == 0) {
            a[i] = k++;
            for(int j = i + i; j < maxn; j += i)
            a[j] = a[i];
        }
    }

    while(~scanf("%d", &N)) {
        printf("%d\n", a[N]);
    }
    return 0;
}