HDU-1796 How many integers can you find 容斥定理

红果果的容斥定理。

代码如下：

#include <cstdlib>

#include <cstring>

#include <cstdio>

#include <algorithm>

using namespace std;



int ans, seq[15], N, flag;



int M;



inline int GCD(int a, int b)

{

    int t;

    while (b) {

        t = a;

        a = b;

        b = t % b;

    }

    return a;

}



inline int LCM(int a, int b) 

{

    return a / GCD(a, b) * b;

}



void dfs(int p, int num, int sign)

{

    if (p > 0) {

        ans += sign * N / num;

    }

    for (int i = p + 1; i <= M; ++i) { 

        dfs(i, LCM(seq[i], num), -sign);

    }

}



int main()

{

    int i, j;

    while (scanf("%d %d", &N, &M) == 2) {

        flag = 0;

        ans = 0; 

        for (i = 1, j = 1; i <= M; ++i, ++j) {

            scanf("%d", &seq[j]);

            if (seq[j] <= 0 || seq[j] >= N) {

                --j;

            }

        }

        N -= 1;

        M = j - 1;

        dfs(0, 1, -1);

        printf("%d\n", ans);

    }    

    return 0;

}