URAL 1039 Anniversary Party 树形dp

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const int maxn=10000;
const int maxm=110000;
int n;

struct EdgeNode{
    int to,next;
}edges[maxm];
int head[maxn],edge;
void init(){
    memset(head,-1,sizeof(head));
    edge=0;
}
void addedge(int u,int v){
    edges[edge].to=v,edges[edge].next=head[u],head[u]=edge++;
}
int a[maxn];
int f[maxn][2];
int id[maxn];
int dfs(int u,int flag){
    if (f[u][flag]!=-1) return f[u][flag];
    int res;
    if (flag){
        res=0;
        for (int i=head[u];i!=-1;i=edges[i].next){
            int v=edges[i].to;
            res+=dfs(v,0);
        }
    }
    else{
        int res1=0,res2=0;
        for (int i=head[u];i!=-1;i=edges[i].next){
            int v=edges[i].to;
            res1+=dfs(v,1);
        }
        for (int i=head[u];i!=-1;i=edges[i].next){
            int v=edges[i].to;
            res2+=dfs(v,0);
        }
        res=max(res1+a[u],res2);
    }
    return f[u][flag]=res;
}
int main(){
    while (~scanf("%d",&n)){
        init();
        memset(f,-1,sizeof(f));
        memset(id,0,sizeof(id));
        for (int i=1;i<=n;i++) scanf("%d",&a[i]);
        int x,y;
        while (~scanf("%d%d",&x,&y)){
            if (x==0&&y==0) break;
            addedge(y,x);
            id[x]++;
        }
        int rt=1;
        for (int i=1;i<=n;i++){
            if (id[i]==0) rt=i;
        }
        printf("%d\n",dfs(rt,0));
    }
	return 0;
}

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