hdu 2068 (Staggered + composition) water title

hdu 2068 (Staggered + composition) water title

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Done once, she did not do it before, this time to do so, the feeling is quite water, plus the combination formula is wrong row on it

错排 official:f[n]=(n-1)*(f[n-1]+f[n-2]);

Combination formula:

c[0]=1;
/**组合公式（递推实现）*/
for(int i=1;i<=n;i++){
	c[i]=c[i-1]*(n-i+1)/i;、
}

Since the topic that long answer to half or more, and then only to the requirement:

Then the following code:

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<iostream>
#define ll long long
using namespace std;
ll f[30];
ll c[30];
int main(){
    f[0]=1;
    f[1]=0;
    f[2]=1;
    /**全错排公式*/
    for(int i=3;i<=25;i++){
        f[i]=(f[i-1]+f[i-2])*(i-1);
    }
    int n;
    while(~scanf("%d",&n)){
        if(n==0)
            break;
        memset(c,0,sizeof(c));
        c[0]=1;
        /**组合公式（递推实现）*/
        for(int i=1;i<=n;i++){
            c[i]=c[i-1]*(n-i+1)/i;
        }
        ll sum=0;
        int m;
        for(int i=0;i<=n/2;i++){
            sum+=c[n-i]*f[i];
        }
        printf("%lld\n",sum);
    }
    return 0;
}

Hdu 2049 which is also a problem related to the wrong row. . . .

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