CF 462D(Appleman and Tree-树形dp)
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题意:一棵n个节点的树(根为0),树上有一些点是黑色的,你希望删除一个边的集合,使每个剩下的联通块都恰好只有1个黑节点。求方案数。
然后转移,考虑x的每个子树是否与x在同一连通块、
#include<bits/stdc++.h> using namespace std;#define For(i,n) for(int i=1;i<=n;i++)#define Fork(i,k,n) for(int i=k;i<=n;i++)#define ForkD(i,k,n) for(int i=n;i>=k;i--)#define Rep(i,n) for(int i=0;i<n;i++)#define ForD(i,n) for(int i=n;i;i--)#define RepD(i,n) for(int i=n;i>=0;i--)#define Forp(x) for(int p=pre[x];p;p=next[p])#define Forpiter(x) for(int &p=iter[x];p;p=next[p]) #define Lson (o<<1)#define Rson ((o<<1)+1)#define MEM(a) memset(a,0,sizeof(a));#define MEMI(a) memset(a,0x3f,sizeof(a));#define MEMi(a) memset(a,128,sizeof(a));#define MEMx(a,b) memset(a,b,sizeof(a));#define INF (0x3f3f3f3f)#define F (1000000007)#define pb push_back#define mp make_pair#define fi first#define se second#define vi vector<int> #define pi pair<int,int>#define SI(a) ((a).size())#define Pr(kcase,ans) printf("Case #%d: %lld\n",kcase,ans);#define PRi(a,n) For(i,n-1) cout<<a[i]<<' '; cout<<a[n]<<endl;#define PRi2D(a,n,m) For(i,n) { \ For(j,m-1) cout<<a[i][j]<<' ';\ cout<<a[i][m]<<endl; \ } #pragma comment(linker, "/STACK:102400000,102400000")#define ALL(x) (x).begin(),(x).end()#define gmax(a,b) a=max(a,b);#define gmin(a,b) a=min(a,b);typedef long long ll;typedef long double ld;typedef unsigned long long ull;ll mul(ll a,ll b){return (a*b)%F;}ll add(ll a,ll b){return (a+b)%F;}ll sub(ll a,ll b){return ((a-b)%F+F)%F;}void upd(ll &a,ll b){a=(a%F+b%F)%F;}inline int read(){ int x=0,f=1; char ch=getchar(); while(!isdigit(ch)) {if (ch=='-') f=-1; ch=getchar();} while(isdigit(ch)) { x=x*10+ch-'0'; ch=getchar();} return x*f;} #define MAXN (112345)int n,col[MAXN];vi e[MAXN];int fa[MAXN];ll dp[MAXN][2];ll pow2(ll a,ll b) { if (!b) return 1%F; if(b==1) return a%F; ll p=pow2(a,b/2); p=mul(p,p); if(b&1) p=p*a%F; return p;}ll inv(ll a){ return pow2(a,F-2);}void dfs(int x){ for(auto v:e[x]) dfs(v); if(col[x]) { dp[x][0]=0; dp[x][1]=1; for(auto v:e[x]) { dp[x][1]=dp[x][1]*(dp[v][0]+dp[v][1])%F; } }else { dp[x][1]=0; dp[x][0]=1; for(auto v:e[x]) { dp[x][0]=dp[x][0]*(dp[v][0]+dp[v][1])%F; } for(auto v:e[x]) { upd(dp[x][1], (ll)dp[x][0]*inv(dp[v][0]+dp[v][1])%F*dp[v][1]%F); } }}int main(){// freopen("D.in","r",stdin);// freopen(".out","w",stdout); n=read();fa[0]=0; For(i,n-1) { fa[i]=read(); e[fa[i]].pb(i); } Rep(i,n) col[i]=read(); dfs(0); cout<<dp[0][1]<<endl; return 0;}
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