bzoj2783: [JLOI2012]树

#include<cstdio>
#include<iostream>
#include<cstring>
#include<cstdlib>
#include<algorithm>
#include<cmath>
#include<set>
using namespace std;

int n,S,c[110000];
struct node
{
    int x,y,next;
}a[110000];int len,last[110000];
void ins(int x,int y)
{
    len++;
    a[len].x=x;a[len].y=y;
    a[len].next=last[x];last[x]=len;
}

int sum;
multiset<int>ss;
void dfs(int x,int d)
{
    if(ss.find(d-S)!=ss.end())sum++;
    ss.insert(d);
        
    for(int k=last[x];k;k=a[k].next)
    {
        int y=a[k].y;
        dfs(y,d+c[y]);
    }
    ss.erase(d);
}
int main()
{
    freopen("a.in","r",stdin);
    freopen("a.out","w",stdout);
    scanf("%d%d",&n,&S);
    for(int i=1;i<=n;i++)scanf("%d",&c[i]);
    
    int x,y;
    len=0;memset(last,0,sizeof(last));
    for(int i=1;i<n;i++)
    {
        scanf("%d%d",&x,&y);
        ins(x,y);
    }
    ss.insert(0);dfs(1,c[1]);
    printf("%d\n",sum);
    return 0;
}