hdu 6327 I. Random Sequence
阿新 • • 發佈:2019-02-20
題解:
定義dp狀態f[N] [gcd(a[i-2],a[i-1],a[i])] [gcd(a[i-1],a[i])] [a[i]]。但是很多狀態是沒有用到的,實際上對於m=100時後面三維加起來也不會超過2000,a[i]是gcd(a[i-1],a[i])整數倍,gcd(a[i-1],a[i])是gcd(a[i-2],a[i-1],a[i])的整數倍。
狀態轉移時列舉新的一維,轉移方程為:
f[N+1][ gcd(a[i-1],a[i],a[i+1])) ] [ gcd(a[i],a[i+1]) ] [ a[i+1] ] += f[N] [gcd(a[i-2],a[i-1],a[i])] [gcd(a[i-1],a[i])] [a[i]] * v[ gcd(a[i-2],a[i-1],a[i],a[i+1] ]。
最後答案除以總的概率的逆元。
方程很長,可以預處理狀態壓縮一下。
#include"bits/stdc++.h" using namespace std; typedef long long LL; int T,n,m; const int mod = 1e9+7; const int N = 105; const int M = 4005; int a[N],v[N],gcd[N][N],id[N][N][N]; int g[M][N],w[M][N]; LL f[N][M]; LL qpow(LL a, LL n) { LL ret = 1; while(n){ if(n&1) ret = ret*a%mod; a = a*a%mod; n >>= 1; } return ret; } void solve() { scanf("%d%d",&n,&m); for(int i = 1; i <= n; i++) scanf("%d",&a[i]); for(int i = 1; i <= m; i++) scanf("%d",&v[i]); for(int i = 1; i <= m; i++) for(int j = 1; j <= m; j++) gcd[i][j] = __gcd(i,j); int cnt = 0; for(int i = 1; i <= m; i++) for(int j = i; j <= m; j+=i) for(int k = j; k <= m; k+=j) id[i][j][k] = ++cnt; //後面三維壓縮狀態 for(int i = 1; i <= m; i++) for(int j = i; j <= m; j+=i) for(int k = j; k <= m; k+=j) { int x = id[i][j][k]; for(int y = 1; y <= m; y++) { g[x][y] = id[gcd[j][y]][gcd[k][y]][y]; //轉移時的下一個狀態 w[x][y] = v[gcd[i][y]]; //轉移時要乘上的權值 } } for(int i = 0; i <= n; i++) for(int j = 0; j <= cnt; j++) f[i][j] = 0; for(int i = 1; i <= m; i++) for(int j = 1; j <= m; j++) for(int k = 1; k <= m; k++) { if(a[1] && a[1] != i) continue; if(a[2] && a[2] != j) continue; if(a[3] && a[3] != k) continue; f[3][ id[gcd[i][gcd[j][k]]][gcd[j][k]][k] ] += 1; } for(int i = 3; i < n; i++) for(int j = 1; j <= cnt; j++) for(int k = 1; k <= m; k++) { if(a[i+1] && a[i+1] != k) continue; int nxt = g[j][k]; f[i+1][nxt] = (f[i+1][nxt]+f[i][j]*w[j][k])%mod; } LL ans = 0; for(int j = 1; j <= cnt; j++) ans = (ans+f[n][j])%mod; for(int i = 1; i <= n; i++) if(!a[i]) ans = ans*qpow(m,mod-2)%mod; cout<<ans<<endl; } int main() { #ifdef LOCAL freopen("input.txt","r",stdin); #endif // LOCAL for(scanf("%d",&T); T; T--) solve(); return 0; }