BZOJ 2783 [JLOI2012]树

#include<iostream>
#include<cstdio>
#include<map>
#include<cstring>
using namespace std;
const int maxn=100009;

int n,k;
int root;
int w[maxn];
int notroot[maxn];

int ans;

int cntedge;
int head[maxn];
int to[maxn<<1],nex[maxn<<1];
void Addedge(int x,int y){
	nex[++cntedge]=head[x];
	to[cntedge]=y;
	head[x]=cntedge;
}

map<int,int>ma;
void Dfs(int x,int fa,int sum){
	if(ma[sum-k]!=0)++ans;
	ma[sum]++;
	for(int i=head[x];i;i=nex[i]){
		if(to[i]==fa)continue;
		Dfs(to[i],x,sum+w[to[i]]);
	}
	ma[sum]--;
}

int main(){
	scanf("%d%d",&n,&k);
	for(int i=1;i<=n;++i)scanf("%d",&w[i]);
	for(int i=1;i<=n-1;++i){
		int x,y;
		scanf("%d%d",&x,&y);
		Addedge(x,y);
		notroot[y]=1;
	}
	for(int i=1;i<=n;++i)if(notroot[i]==0)root=i;
	
	ma[0]=1;
	Dfs(root,0,w[root]);
	
	printf("%d\n",ans);
	return 0;
}