BZOJ 2783 树

Posted 2020-07-07 ziliuziliu

树上倍增。

#include<iostream>
#include<cstdio>
#include<cstring>
#define maxv 100500
#define maxe 200500
using namespace std;
struct edge
{
    int v,nxt;
}e[maxe];
int n,s,w[maxv],dis[maxv],x,y,nume=0,g[maxv],root,anc[maxv][20],sum[maxv][20],ans=0;
bool vis[maxv];
void addedge(int u,int v)
{
    e[++nume].v=v;
    e[nume].nxt=g[u];
    g[u]=nume;
}
void dfs(int now)
{
    for (int i=g[now];i;i=e[i].nxt)
    {
        int v=e[i].v;
        dis[v]=dis[now]+1;
        anc[v][0]=now;
        sum[v][0]=w[now];
        dfs(v);
    }
}
int find(int x)
{
    int regis=w[x],now=x;
    for (int ee=19;ee>=0;ee--)
    {
        if (regis+sum[now][ee]<=s)
        {
            regis+=sum[now][ee];
            now=anc[now][ee];
        }
    }
    if (regis==s) return 1;
    return 0;
}
int main()
{
    scanf("%d%d",&n,&s);
    for (int i=1;i<=n;i++)
        scanf("%d",&w[i]);
    for (int i=1;i<=n-1;i++)
    {
        scanf("%d%d",&x,&y);
        addedge(x,y);
        vis[y]=true;
    }    
    for (int i=1;i<=n;i++)
    {
        if (vis[i]==false)
            root=i;
    }
    dfs(root);
    for (int ee=1;ee<=19;ee++)
        for (int i=1;i<=n;i++)
        {
            anc[i][ee]=anc[anc[i][ee-1]][ee-1];
            sum[i][ee]=sum[anc[i][ee-1]][ee-1]+sum[i][ee-1];
        }
    for (int i=1;i<=n;i++)
        ans+=find(i);
    printf("%d\n",ans);
    return 0;
}