Almost Prime

Description

Almost Prime

time limit per test: 2 seconds

memory limit per test: 256 megabytes

input: standard input

output: standard output

A number is called almost prime if it has exactly two distinct prime divisors. For example, numbers 6, 18, 24 are almost prime, while 4, 8, 9, 42 are not. Find the amount of almost prime numbers which are between 1 and n, inclusive.

Input

Input contains one integer number n (1 ≤ n ≤ 3000).

Output

Output the amount of almost prime numbers between 1 and n, inclusive.

Sample test(s)

input

10

output

2

 

 

 

 

 

 

input

21

output

8

 

 

 

 

 

 

题解：

　　素因子数的种类有且只有两种，称为Almost Prime。

       例如：18=2*3*3，只有2和3，是Almost Prime。

       求1~n Almost Prime 数量。

代码：

#include <iostream>
#include <cstdio>

using namespace std;
//计算数字n不同素因子个数
int distinctPrime (int n);

int main()
{
    int n, i, counter = 0;
    cin >> n;
    for(i=1; i<=n; i++){
        if(distinctPrime(i) == 2)
            counter ++;
    }
    cout << counter <<endl;
    return 0;
}

int distinctPrime (int n) {
    int i, t, flag, counter = 0;
    for(i=2, t=n; i<n; i++) {
        while(t>0) {
            if(t%i==0)
                t = t/i;
            else
                break;
            flag = 1;
        }
        if(flag)
            counter++;
        flag = 0;
    }
    return counter;
}

 

Problem -26A - Codeforces

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