hdu1787——GCD Again

Do you have spent some time to think and try to solve those unsolved problem after one ACM contest?
No? Oh, you must do this when you want to become a “Big Cattle”.
Now you will find that this problem is so familiar:
The greatest common divisor GCD (a, b) of two positive integers a and b, sometimes written (a, b), is the largest divisor common to a and b. For example, (1, 2) =1, (12, 18) =6. (a, b) can be easily found by the Euclidean algorithm. Now I am considering a little more difficult problem:
Given an integer N, please count the number of the integers M(0<M<N)which satisfies (N,M)>1.
This is a simple version of problem “GCD” which you have done in a contest recently,so I name this problem “GCD Again”.If you cannot solve it still,please take a good think about your method of study.
Good Luck!
Input
Input contains multiple test cases. Each test case contains an integers N(1<N<100000000). A test case containing 0 terminates the input and this test case is not to be processed.
Output
For each integers N you should output the number of integers M in one line, and with one line of output for each line in input.
Sample Input
2
4
0
Sample Output
0
1

这题以为是素数，结果数组没办法开那么打，原来是欧拉函数

代码：

#include <cstdio>
#include <algorithm>
#include <cstring>
using namespace std;
const int N=100000020;
int Euler(int n){
    int res=n;
    int a=n;
    for(int i=2;i*i<=a;i++){
        if(a%i==0){
            //每个因子计算一次
            res=res/i*(i-1);
            while(a%i==0){
                a/=i;
            }
        }
    }
    if(a>1){
        res=res/a*(a-1);
    }
    return res;
}
int main(void){
    int n;
    while(~scanf("%d",&n) && n){
        printf("%d\n",n-1-Euler(n));
    }
    return 0;
}