SP2829 TLE (FWT)

Posted 2021-03-10 hs-black

高维前缀和

众所周知, FWT可以轻松的算出高维前缀和

本题题解:

考虑状压(dp)(题目都说了(2^M)那就状压了)因为(\\%c[i])和(&a[i-1])这两个操作都和具体的数值有关

(F[i][j])表示枚举到(i), 第(i)个数填(j)有多少种方案
[ F[i][j] = egin{cases} 0~~~~~~~~~~~ j~\\%~c[i] ==0\\\\displaystyle sum_{k&j=0} F[i-1][j]end{cases} ]
这样显然是不行的, 需要一点优化

转移时发现每一个合法的k都是(j igotimes (2^M-1))的子集, 如果能记个子集前缀和那就再好不过了, FWT可以帮我们完成这件事, 一遍FWT_or算出子集和, 给出一份易于背诵的代码吧 戳这里

完整代码

#include <iostream>
#include <cstdio>
#include <cstring>
#define ll long long
using namespace std;
const int P = 1000000000;
template <typename T>
void read(T &x) {
    x = 0; bool f = 0;
    char c = getchar();
    for (;!isdigit(c);c=getchar()) if (c=='-') f=1;
    for (;isdigit(c);c=getchar()) x=x*10+(c^48);
    if (f) x=-x;
}

ll ans = 0, n, m;
void FWT_or(ll *f) {
    for (int i = 1, p = 2; i < m; i <<= 1, p <<= 1) 
        for (int j = 0; j < m; j += p) 
            for (int k = 0; k < i; k++) 
                (f[i+j+k] += f[j+k]) %= P;
}

const int N = 55;
ll f[N][(1<<15) + 5], a[N];

int main() {
    int T; read(T);
    while (T--) {
        memset(f, 0, sizeof(f));
        read(n), read(m); m = 1 << m;
        for (int i = 1; i <= n; i++) read(a[i]);
        for (int i = 1; i < m; i++) 
            if (i % a[1]) f[1][i] = 1;
        FWT_or(f[1]);
        for (int i = 2; i <= n; i++) {
            for (int j = 1; j < m; j++) {
                if (j % a[i] == 0) continue;
                f[i][j] += f[i-1][(m-1)^j];
            }
            FWT_or(f[i]);
        }
        cout << f[n][m-1] << endl;
    }
    return 0;
}