2019HDU多校第九场 Rikka with Quicksort —— 数学推导&&分段打表

题意

设

$$g_m(n)=\begin{cases}
& g_m(i) = 0,     \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 0 \leq i \leq m\\
& g_m(i) = i-1 + \frac{1}{i}\sum _{j=1}^i(g_m(j) + g_m(i-j)), \ \  i > m\\
\end{cases}$$

现给出 $n$ 和 $m$，求 $g_m(n)$ 模 $1000000007$.

分析

当 $n>m$ 时，易知 $a_n = n-1 + \frac{2}{n} S_{n-1}$，

即 $S_n - S_{n-1} = n-1 + \frac{2}{n} S_{n-1}$，

变形得 $\frac{S_n}{(n+1)(n+2)} = \frac{n-1}{(n+1)(n+2)} + \frac{S_{n-1}}{n(n+1)}$.

令 $b_n = \frac{S_n}{(n+1)(n+2)}$，得 $b_n = \frac{n-1}{(n+1)(n+2)} + b_{n-1}$，

变形 $b_n - b_{n-1} = \frac{3}{n+2} - \frac{2}{n+1} = 2(\frac{1}{n+2}-\frac{1}{n+1}) + \frac{1}{n+2}$，

根据裂项相消得 $b_n - b_m = 2(\frac{1}{n+2} - \frac{1}{m+2}) + \frac{1}{m+3} + \frac{1}{m+4}+...+\frac{1}{n+2}$，

即 $b_n = 2(\frac{1}{n+2} - \frac{1}{m+2}) + S(n+2)-S(m+2)$.

其中 $S(n)$ 表示前 $n$ 个倒数和，可以分段打表。

（ $1e4$ 的表虽然不会超内存限制，但是会超代码长度限制www，最后改成2000

#include<bits/stdc++.h>
using namespace std;

typedef long long ll;
const ll mod = ;
const int maxn = 1e9 + ;
const int siz = ;  //
int f[maxn/siz+] = {{,,,,,,,...}  //省略了
ll n, m;

ll qpow(ll a, ll b, ll p)
{
    ll ret = ;
    while(b)
    {
        if(b&)  ret = ret * a % p;
        a = a * a % p;
        b >>= ;
    }
    return ret;
}

ll inv(ll n)
{
    return qpow(n, mod-, mod);
}

ll S(ll n)
{
    int k = n / siz; //printf("%d\n", k);
    ll tmp = f[k];
    for(int i = k*siz+;i <= n;i++)  tmp = (tmp + inv(i)) % mod;
    return tmp;
}

int main()
{
    int T;
    scanf("%d", &T);
    while(T--)
    {
        scanf("%lld%lld", &n, &m);
        ll bn = (*(inv(n+) - inv(m+)) + S(n+) - S(m+)) % mod;
        ll bn_1 = (*(inv(n+) - inv(m+)) + S(n+) - S(m+)) % mod;
        ll sn = (n+) * (n+) % mod * bn % mod;
        ll sn_1 = n * (n+) % mod * bn_1 % mod;
        printf("%lld\n", (sn - sn_1 + *mod) % mod);
    }
}

参考链接：https://blog.csdn.net/baiyifeifei/article/details/99892190