Codeforces Round #277 (Div. 2) D. Valid Sets DP
As you know, an undirected connected graph with n nodes and n - 1 edges is called a tree. You are given an integer d and a tree consisting of n nodes. Each node i has a value ai associated with it.
We call a set S of tree nodes valid if following conditions are satisfied:
- S is non-empty.
- S is connected. In other words, if nodes u and v are in S, then all nodes lying on the simple path between u and v should also be presented in S.
- .
Your task is to count the number of valid sets. Since the result can be very large, you must print its remainder modulo 1000000007(109 + 7).
The first line contains two space-separated integers d (0 ≤ d ≤ 2000) and n (1 ≤ n ≤ 2000).
The second line contains n space-separated positive integers a1, a2, ..., an(1 ≤ ai ≤ 2000).
Then the next n - 1 line each contain pair of integers u and v (1 ≤ u, v ≤ n) denoting that there is an edge between u and v. It is guaranteed that these edges form a tree.
Print the number of valid sets modulo 1000000007.
1 4
2 1 3 2
1 2
1 3
3 4
8
In the first sample, there are exactly 8 valid sets: {1}, {2}, {3}, {4}, {1, 2}, {1, 3}, {3, 4} and {1, 3, 4}. Set {1, 2, 3, 4} is not valid, because the third condition isn't satisfied. Set {1, 4} satisfies the third condition, but conflicts with the second condition.
题意:给你一个n点的树,和每个点的权值,问你多少种子树满足(最大权值点-最小权值点)<=d
题解:定义dp[i]表示以i为最小权值根节点的子树方案数,注意维护此条件
于是答案就是 ∑dp[i] %mod (1<=i<=n);
///
#include<bits/stdc++.h>
using namespace std ;
typedef long long ll;
#define mem(a) memset(a,0,sizeof(a))
#define pb push_back
#define meminf(a) memset(a,127,sizeof(a)); inline ll read()
{
ll x=,f=;char ch=getchar();
while(ch<''||ch>''){
if(ch=='-')f=-;ch=getchar();
}
while(ch>=''&&ch<=''){
x=x*+ch-'';ch=getchar();
}return x*f;
}
//****************************************
#define maxn 2000+50
#define mod 1000000007
#define inf 1000000007
int d,n,a[maxn],vis[maxn];
vector<int >G[maxn];
ll dp[maxn];//以i为最小根节点,的方案数
void dfs(int x,int pre){
dp[x]=;vis[x]=;
for(int i=;i<G[x].size();i++){
if(!vis[G[x][i]]){
if(a[G[x][i]]<a[pre]||a[G[x][i]]>a[pre]+d)continue;
if(a[G[x][i]]==a[pre]&&G[x][i]<pre)continue;
dfs(G[x][i],pre);
dp[x]=(dp[x]*(dp[G[x][i]]+))%mod;
}
}
} int main(){
d=read(),n=read();
for(int i=;i<=n;i++){
scanf("%d",&a[i]);
}int u,v;
for(int i=;i<n;i++){
scanf("%d%d",&u,&v);
G[u].pb(v);G[v].pb(u);
}ll ans=;
for(int i=;i<=n;i++){
mem(dp);mem(vis);
dfs(i,i);
ans=(ans+dp[i])%mod;
}
cout<<ans<<endl;
return ;
}
代码
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