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客官°小女子只卖身不卖艺 2023-06-03 02:53 32阅读 0赞

## D.Interesting Series ##

F(n)实际上是一个等比数列的和，将它从递推式转变为通项公式(a^n-1)/(a-1)，这里只需要确定n就可以。

题目要求选取k大小的所有子集的答案求和，可以先求a^n部分的和，把它写成母函数的形式(x+a^s1)(x+a^s2)...(x+a^sn)，这样不断的分成两半，分别求出，然后再用fft卷积合并这两半，这一部分的和就是x^(n-k)的系数。

分母的a-1实际上还是a-1，分子的-1部分要变成C(n,k)

由于之前没怎么写过，fft的板子代码借鉴群里大佬的。

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#include <iostream>
    #include <cstdio>
    #include <string>
    #include <cstring>
    #include <algorithm>
    #include <cmath>
    #include <stack>
    #include <vector>
    #include <set>
    #include <cmath>
    #include <queue>
    #include <map>
    #include <cassert>
    const int maxn = 1e5 + 10;
    const int mod = 1e5 + 3;
    using namespace std;
    typedef long long ll;
    const double PI = acos(-1.0);
    namespace fft
    {
    struct num
    {
        double x, y;
        num()
        {
            x = y = 0;
        }
        num(double x, double y) : x(x), y(y) {}
    };
    inline num operator+(num a, num b)
    {
        return num(a.x + b.x, a.y + b.y);
    }
    inline num operator-(num a, num b)
    {
        return num(a.x - b.x, a.y - b.y);
    }
    inline num operator*(num a, num b)
    {
        return num(a.x * b.x - a.y * b.y, a.x * b.y + a.y * b.x);
    }
    inline num conj(num a)
    {
        return num(a.x, -a.y);
    }
    
    int base = 1;
    vector<num> roots = {
         {
         0, 0}, {
         1, 0}};
    vector<int> rev = {
         0, 1};
    const double PI = acosl(-1.0);
    
    void ensure_base(int nbase)
    {
        if (nbase <= base)
            return;
        rev.resize(1 << nbase);
        for (int i = 0; i < (1 << nbase); i++)
            rev[i] = (rev[i >> 1] >> 1) + ((i & 1) << (nbase - 1));
        roots.resize(1 << nbase);
        while (base < nbase)
        {
            double angle = 2 * PI / (1 << (base + 1));
            for (int i = 1 << (base - 1); i < (1 << base); i++)
            {
                roots[i << 1] = roots[i];
                double angle_i = angle * (2 * i + 1 - (1 << base));
                roots[(i << 1) + 1] = num(cos(angle_i), sin(angle_i));
            }
            base++;
        }
    }
    
    void fft(vector<num> &a, int n = -1)
    {
        if (n == -1)
            n = a.size();
        assert((n & (n - 1)) == 0);
        int zeros = __builtin_ctz(n);
        ensure_base(zeros);
        int shift = base - zeros;
        for (int i = 0; i < n; i++)
            if (i < (rev[i] >> shift))
                swap(a[i], a[rev[i] >> shift]);
        for (int k = 1; k < n; k <<= 1)
        {
            for (int i = 0; i < n; i += 2 * k)
            {
                for (int j = 0; j < k; j++)
                {
                    num z = a[i + j + k] * roots[j + k];
                    a[i + j + k] = a[i + j] - z;
                    a[i + j] = a[i + j] + z;
                }
            }
        }
    }
    
    vector<num> fa, fb;
    
    vector<int> multiply_mod(vector<int> &a, vector<int> &b, int m, int eq = 0)
    {
        int need = a.size() + b.size() - 1;
        int nbase = 0;
        while ((1 << nbase) < need)
            nbase++;
        ensure_base(nbase);
        int sz = 1 << nbase;
        if (sz > (int)fa.size())
            fa.resize(sz);
        for (int i = 0; i < (int)a.size(); i++)
        {
            int x = (a[i] % m + m) % m;
            fa[i] = num(x & ((1 << 15) - 1), x >> 15);
        }
        fill(fa.begin() + a.size(), fa.begin() + sz, num{
         0, 0});
        fft(fa, sz);
        if (sz > (int)fb.size())
            fb.resize(sz);
        if (eq)
            copy(fa.begin(), fa.begin() + sz, fb.begin());
        else
        {
            for (int i = 0; i < (int)b.size(); i++)
            {
                int x = (b[i] % m + m) % m;
                fb[i] = num(x & ((1 << 15) - 1), x >> 15);
            }
            fill(fb.begin() + b.size(), fb.begin() + sz, num{
         0, 0});
            fft(fb, sz);
        }
        double ratio = 0.25 / sz;
        num r2(0, -1), r3(ratio, 0), r4(0, -ratio), r5(0, 1);
        for (int i = 0; i <= (sz >> 1); i++)
        {
            int j = (sz - i) & (sz - 1);
            num a1 = (fa[i] + conj(fa[j]));
            num a2 = (fa[i] - conj(fa[j])) * r2;
            num b1 = (fb[i] + conj(fb[j])) * r3;
            num b2 = (fb[i] - conj(fb[j])) * r4;
            if (i != j)
            {
                num c1 = (fa[j] + conj(fa[i]));
                num c2 = (fa[j] - conj(fa[i])) * r2;
                num d1 = (fb[j] + conj(fb[i])) * r3;
                num d2 = (fb[j] - conj(fb[i])) * r4;
                fa[i] = c1 * d1 + c2 * d2 * r5;
                fb[i] = c1 * d2 + c2 * d1;
            }
            fa[j] = a1 * b1 + a2 * b2 * r5;
            fb[j] = a1 * b2 + a2 * b1;
        }
        fft(fa, sz);
        fft(fb, sz);
        vector<int> res(need);
        for (int i = 0; i < need; i++)
        {
            ll aa = fa[i].x + 0.5;
            ll bb = fb[i].x + 0.5;
            ll cc = fa[i].y + 0.5;
            res[i] = (aa + ((bb % m) << 15) + ((cc % m) << 30)) % m;
        }
        return res;
    }
    vector<int> square_mod(vector<int> &a, int m)
    {
        return multiply_mod(a, a, m, 1);
    }
    }; // namespace fft
    inline int quick(int a, int b, int m)
    {
        int ans = 1;
        while (b)
        {
            if (b & 1)
            {
                ans = (1LL * a * ans) % m;
            }
            a = (1LL * a * a) % m;
            b >>= 1;
        }
        return ans;
    }
    vector<int> v[maxn];
    inline ll inverse(ll a, ll p) { return quick(a, p - 2, p); }
    vector<int> solve(int l, int r)
    {
        if (l == r)
            return v[l];
        int mid = (l + r) >> 1;
        vector<int> v1 = (solve(l, mid));
        vector<int> v2 = solve(mid + 1, r);
        return fft::multiply_mod(v1, v2, mod);
    }
    int comb[maxn];
    int main()
    {
        int n, a, q;
        scanf("%d%d%d", &n, &a, &q);
        comb[0] = 1;
        for (int i = 1; i <= n; i++)
        {
            comb[i] = 1LL * comb[i - 1] * (n + 1 - i) * inverse(i, mod) % mod;
        }
        for (int i = 1, s; i <= n; i++)
        {
            scanf("%d", &s);
            v[i].push_back(quick(a % mod, s, mod));
            v[i].push_back(1);
        }
        vector<int> ans = solve(1, n);
        while (q--)
        {
            int k;
            scanf("%d", &k);
            int cnt = 1LL * (ans[n - k] - comb[k]) * inverse(a - 1, mod) % mod;
            if (cnt < 0)
                cnt += mod;
            printf("%d\n", cnt);
        }
        return 0;
    }

转载于:https://www.cnblogs.com/hyfer/p/11492428.html

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