数学--数论--HDU2136 Largest prime factor 线性筛法变形

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.

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

Output the LPF(n).

1 2 3 4 5

0 1 2 1 3

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define N 1000005
int prime[N], cnt;
bool vis[N];
int num[N], e[N], tot = 1;
void init()
{
    for (int i = 2; i <= N; ++i)
    {
        if (!vis[i])
        {
            prime[++cnt] = i;
            num[i] = i + 1;
            e[i] = tot++;
        }
        for (int j = 1; j <= cnt; ++j)
        {
            if (prime[j] * i > N)
                break;
            vis[prime[j] * i] = true;
            e[prime[j] * i] = e[i];
        }
    }
}
int main()
{
    init();

int n;
    while (scanf("%d", &n) != EOF)
    {
        printf("%lld\n", e[n]);
    }
}