51nod1259 整数划分V2

整数划分有个用五边形数定理$O(n\sqrt n)$的做法，粘发链接跑
传送门

code：

#include<set>
#include<map>
#include<deque>
#include<queue>
#include<stack>
#include<cmath>
#include<ctime>
#include<bitset>
#include<string>
#include<vector>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<climits>
#include<complex>
#include<iostream>
#include<algorithm>
#define ll long long
#define pb push_back
using namespace std;

const int maxn = 51000;
const int mod  = 1e9+7;

int n,m;
int f[maxn],g[maxn];

int main()
{
    //freopen("tmp.in","r",stdin);
    //freopen("tmp.out","w",stdout);

    scanf("%d",&n);
    m=0;
    for(int i=1;g[m]<=n;i++)
    {
        g[++m]=i*(3*i-1)/2;
        g[++m]=i*(3*i+1)/2;
    }
    f[0]=1;
    for(int i=1;i<=n;i++)
    {
        for(int j=1;g[j]<=i;j++)
        {
            int sig=(j-1>>1&1)?-1:1;
            (f[i]+=sig*f[i-g[j]])%=mod;
        }
        (f[i]+=mod)%=mod;
    }
    printf("%d\n",f[n]);

    return 0;
}