Jisoo

不显然的树上dp

定义$f_{i,j}$为第i位和为子树所选奇数个/偶数个点的方案数

然后显然会发现奇数加奇数等于偶数等规律

然后就可以转移了

#include<cstdio>
#include<iostream>
#include<cstring>
#include<iomanip>
#include<cmath>
#include<algorithm>
#define int long long
using namespace std;
template<class T>inline void read(T &x)
{
    x=0;register char c=getchar();register bool f=0;
    while(!isdigit(c))f^=c=='-',c=getchar();
    while(isdigit(c))x=(x<<3)+(x<<1)+(c^48),c=getchar();
    if(f)x=-x;
}
template<class T>inline void print(T x)
{
    if(x<0)putchar('-'),x=-x;
    if(x>9)print(x/10);
    putchar('0'+x%10);
}
int f[2000005][2];
int n;
struct e{
    int to;
    int ne;
}ed[200001];
int a[2000005];
int head[2000005];
int x,y;
int p;
void add(int f,int to){
    p++;
    ed[p].ne=head[f];
    ed[p].to=to;
    head[f]=p;
}
void dfs(int ro){
    f[ro][1]=-999999999;
    for(int i=head[ro];i;i=ed[i].ne){
        int v=ed[i].to;
        //if(v==fa) continue;
        dfs(v);
        int x=f[ro][0],y=f[ro][1];
        f[ro][0]=max(x+f[v][0],y+f[v][1]);
        f[ro][1]=max(x+f[v][1],y+f[v][0]);
    }
    f[ro][1]=max(f[ro][1],f[ro][0]+a[ro]);
}
signed main(){
    ios::sync_with_stdio(false);
    read(n);
    for(int i=1;i<=n;++i){
        read(x);
        read(a[i]);
        if(x!=-1) 
        add(x,i);
    }
    //memset(f,0xc0,sizeof(f));
    dfs(1);

    cout<<max(f[1][0],f[1][1]);
    return 0;
}