BZOJ 2783 [JLOI2012]树【倍增一下】

#include <cmath>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#define db double
#define sg string
#define ll long long
#define rep(i,x,y) for(ll i=(x);i<=(y);i++)
#define red(i,x,y) for(ll i=(x);i>=(y);i--)
#define res(i,x) for(ll i=head[x];i;i=nxt[i])
using namespace std;
 
const ll N=1e5+5;
const ll Inf=1e18;
 
ll n,s,ans,v[N],dis[N],fa[N][21];
ll cnt,to[N<<1],nxt[N<<1],edge[N<<1],head[N];
 
inline ll read() {
    ll x=0;char ch=getchar();bool f=0;
    while(ch>'9'||ch<'0'){if(ch=='-')f=1;ch=getchar();}
    while(ch>='0'&&ch<='9'){x=(x<<1)+(x<<3)+ch-'0';ch=getchar();}
    return f?-x:x;
}
 
void ins(ll x,ll y,ll z) {
    to[++cnt]=y;edge[cnt]=z;nxt[cnt]=head[x];head[x]=cnt;
}
 
void dfs(ll x) {
    rep(i,1,19) fa[x][i]=fa[fa[x][i-1]][i-1];
     
    res(i,x) {
        ll y=to[i],z=edge[i];
        if(y!=fa[x][0]) {
            dis[y]=dis[x]+z;fa[y][0]=x;dfs(y);
        }
    }
} 
 
void getans(ll x) {
    ll tmp=x;
     
    red(i,20,0) if(dis[x]-dis[fa[tmp][i]]<=s) tmp=fa[tmp][i];
     
    if (dis[x]-dis[tmp]==s) ans++; 
     
    res(i,x) getans(to[i]);
}
 
void File() {
    freopen("tree.in","r",stdin);
    freopen("tree.out","w",stdout);
}
 
int main() {
//  File();
     
    n=read(),s=read();
     
    rep(i,1,n) v[i]=read();
     
    rep(i,2,n) {
        ll x=read(),y=read();ins(x,y,v[y]);
    }
     
    dis[1]=v[1];
     
    dfs(1);
     
    getans(1);
     
    printf("%lld\n",ans);
 
    return 0;
}