【cf755G】G. PolandBall and Many Other Balls(dp+生成函数+倍增)

传送门

题意:
给定(n)个小球,标号为(1,2,...,n),现在要从中选出(m)组。每一组只能包含一个小球或者两个标号相邻的小球。
给定(k),求出所有分组为(m,1leq mleq k)的方案数。

思路:
考虑直接暴力(dp:dp_{i,j})表示前(i)个球划分(j)个组的方案数,那么考虑第(i)个数划分的三种情况:

  • 不加入任意一组中,那么(dp_{i,j}=dp_{i-1,j});
  • (i-1)合并为一组,那么此时就有(dp_{i,j}=dp_{i-2,j-1});
  • 自己单独为一组,那么(dp_{i,j}=dp_{i-1,j-1})

但这样(dp)时空显然不能承受,观察上面(dp)转移式,我们相当于从最近的一处断开,其实我们可以考虑倍增进行(dp),也就是从中间断开。
我们定义(F_i)为一个(i)次多项式(其实就是生成函数),每一项系数为将(i)个数划分(j)组的方案数。
假设现在知道(F_a,F_{a-1},F_{a-2}),那么我们容易推出相关式子:

  • 从中间断开,即(a,a+1)不在同一组中:(F_{a+a}=F_a*F_a);
  • (a,a+1)为一组时:(F_{a+a}=x* F_{a-1}*F_{a-1});

(F_{2a-1},F_{2a-2})同理。

但这样只能做(n)(2)的次幂的情况,我们考虑求(F_{a+1}:)

  • (displaystyle F_{a+1}=2F_{a}+xF_{a-1})

这样我们即可类似于快速幂那样求出(n)为任意数的值。

现在我们已知(F_{a-2},F_{a-1},F_a),就可以推得(F_{2a-2},F_{2a-1},F_{2a})
并且也可以通过(F_{a-2},F_{a-1},F_{a})得到(F_{a-1},F_{a},F_{a+1})
这个题就愉快地做出来了(好难)
代码如下:

/*
 * Author:  heyuhhh
 * Created Time:  2020/4/10 10:46:08
 */
#include <iostream>
#include <algorithm>
#include <cstring>
#include <cstdio>
#include <vector>
#include <cmath>
#include <set>
#include <map>
#include <queue>
#include <iomanip>
#include <assert.h>
#define MP make_pair
#define fi first
#define se second
#define pb push_back
#define sz(x) (int)(x).size()
#define all(x) (x).begin(), (x).end()
#define INF 0x3f3f3f3f
#define Local
#ifdef Local
  #define dbg(args...) do { cout << #args << " -> "; err(args); } while (0)
  void err() { std::cout << std::endl; }
  template<typename T, typename...Args>
  void err(T a, Args...args) { std::cout << a << ' '; err(args...); }
  template <template<typename...> class T, typename t, typename... A> 
  void err(const T <t> &arg, const A&... args) {
  for (auto &v : arg) std::cout << v << ' '; err(args...); }
#else
  #define dbg(...)
#endif
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
//head
const int MOD = 998244353, inv2 = (MOD + 1) >> 1;
const int N = 4e5 + 5, M = (1 << 19); //四倍空间,M=2^x且M>=N
typedef vector <int> Poly;
#define ri register int
#define cs const
inline int add(int a, int b) {return a + b >= MOD ? a + b - MOD : a + b;}
inline int dec(int a, int b) {return a < b ? a - b + MOD : a - b;}
inline int mul(int a, int b) {return 1ll * a * b % MOD;}
inline int qpow(int a, int b) {int res = 1; while(b) {if(b & 1) res = mul(res, a); a = mul(a, a); b >>= 1;} return res;}
int rev[N], inv[N], pw[M], W[2][M];
inline void init() {
    inv[0] = inv[1] = 1;
    for(ri i = 2; i < N; i++) inv[i] = mul(inv[MOD % i], MOD - MOD / i);
    int w0 = qpow(3, (MOD - 1) / M), w1 = qpow((MOD + 1) / 3, (MOD - 1) / M);
    W[0][0] = W[1][0] = 1;
    for(ri i = 1; i < M; i++) {
        W[0][i] = mul(W[0][i - 1], w0);
        W[1][i] = mul(W[1][i - 1], w1);
    }
}      
inline void init_rev(int len) {
    for(ri i = 0; i < len; i++) rev[i] = rev[i >> 1] >> 1 | ((i & 1) * (len >> 1));   
}
inline void NTT(Poly &a, int n, int op) {
    for(ri i = 0; i < n; i++) if(i < rev[i]) swap(a[i], a[rev[i]]);
    for(ri i = 1; i < n; i <<= 1) {
        int t = M / (i << 1), t2 = (op == -1 ? 1 : 0);
        for(ri j = 0; j < i; ++j) pw[j] = W[t2][j * t];
        int wn = (op == -1 ? W[1][i] : W[0][i]);
        for(ri j = 0; j < n; j += (i << 1)) {
            for(ri k = 0, x, y; k < i; ++k) {
                x = a[j + k], y = mul(pw[k], a[j + k + i]);
                a[j + k] = add(x, y);
                a[j + k + i] = dec(x, y);
            }   
        }
    }   
    if(op == -1) for(ri i = 0; i < n; i++) a[i] = mul(a[i], inv[n]);
}
inline Poly operator + (cs Poly &a, cs Poly &b) {
    Poly c = a; c.resize(max(sz(a), sz(b)));
    for(ri i = 0; i < sz(b); i++) c[i] = add(c[i], b[i]);
    return c;   
}
inline Poly operator * (Poly a, Poly b) {
    int n = sz(a), m = sz(b), l = 1;
    while(l < n + m - 1) l <<= 1;
    init_rev(l);
    a.resize(l), NTT(a, l, 1);
    b.resize(l), NTT(b, l, 1);
    for(ri i = 0; i < l; i++) a[i] = mul(a[i], b[i]);
    NTT(a, l, -1);
    a.resize(n + m - 1);
    return a;
}

void move(Poly &t) {
    t.resize(sz(t) + 1);
    for(int i = sz(t) - 1; i; i--) t[i] = t[i - 1];
    t[0] = 0;      
}

vector <Poly> add(cs vector <Poly> &f) {
    Poly t = f[2] + f[1];
    move(t);
    return vector <Poly> {f[1], f[2], f[2] + t};
}

Poly combine(Poly &a, Poly &b, Poly &c, Poly &d) {
    Poly A = a * b, B = c * d;
    move(B);
    Poly ans = A + B;
    return ans;
}

vector <Poly> combine(vector <Poly> &f) {
    vector <Poly> g(3);
    g[2] = combine(f[2], f[2], f[1], f[1]);
    g[1] = combine(f[2], f[1], f[1], f[0]);
    g[0] = combine(f[1], f[1], f[0], f[0]);
    return g;
}

void run() {
    init();
    int n, k; cin >> n >> k;
    vector <Poly> f(3);
    f[0] = vector <int> {1};
    f[1] = vector <int> {1, 1};
    f[2] = vector <int> {1, 3, 1};
    int high;
    for(high = 31; high >= 0; high--) if(n >> high & 1) break;
    for(--high; high >= 0; high--) {
        if(n >> high & 1) f = add(f);
        if(high) {
            vector <Poly> g = combine(f);
            f.swap(g);
        }
        for(int i = 0; i < 3; i++) f[i].resize(k + 1);
    } 
    int o = (n == 1 ? 1 : 2);
    for(int i = 1; i <= k; i++) {
        cout << f[o][i] << " 
"[i == k];
    }
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0); cout.tie(0);
    cout << fixed << setprecision(20);
    run();
    return 0;
}
原文地址:https://www.cnblogs.com/heyuhhh/p/12723253.html