思路：二分答案之后，树形dp去跑每个节点的取值范围就好了
#include<bits/stdc++.h>
using namespace std;
const int maxn = 5e4+6;
int w[maxn],n,k,l[maxn],r[maxn],vis[maxn];
vector<int>E[maxn];
int dfsl(int x,int d)
{
for(int i=0;i<E[x].size();i++)
{
if(!vis[E[x][i]])
{
vis[E[x][i]]=1;
l[x]=max(l[x],dfsl(E[x][i],d));
}
}
return l[x]-d;
}
int dfsr(int x,int d)
{
for(int i=0;i<E[x].size();i++)
{
if(!vis[E[x][i]])
{
vis[E[x][i]]=1;
r[x]=min(r[x],dfsr(E[x][i],d));
}
}
return r[x]+d;
}
bool check(int mid)
{
for(int i=1;i<=n;i++)
if(w[i])l[i]=r[i]=w[i];
else l[i]=0,r[i]=1e9;
memset(vis,0,sizeof(vis));
dfsl(1,mid);
memset(vis,0,sizeof(vis));
dfsr(1,mid);
for(int i=1;i<=n;i++)
if(r[i]<l[i])return false;
return true;
}
void solve()
{
for(int i=0;i<maxn;i++)E[i].clear();
for(int i=0;i<maxn;i++)w[i]=0;
scanf("%d%d",&n,&k);
for(int i=1;i<n;i++)
{
int x,y;scanf("%d%d",&x,&y);
E[x].push_back(y);
E[y].push_back(x);
}
for(int i=1;i<=k;i++)
{
int x,y;
scanf("%d%d",&x,&y);
w[x]=y;
}
int L=0,R=1e9,ans=1e9;
while(L<=R)
{
int mid=(L+R)/2;
if(check(mid))R=mid-1,ans=mid;
else L=mid+1;
}
cout<<ans<<endl;
}
int main()
{
int t;scanf("%d",&t);
while(t--)solve();
return 0;
}
Problem Description
n nodes, including
(n−1) undirected edges, whose nodes are numbered from
1 to
n. The degree of each node is defined as the number of the edges connected to it, and each node whose degree is
1 is defined as the leaf node of the tree.
zxa wanna set each node's beautiful level, which must be a positive integer. His unrooted tree has
m(1≤m≤n) leaf nodes,
k(1≤k≤m) leaf nodes of which have already been setted their beautiful levels, so that zxa only needs to set the other nodes' beautiful levels.
zxa is interested to know, assuming that the ugly level of each edge is defined as the absolute difference of the beautiful levels between two nodes connected by this edge, and the ugly level of the tree is the maximum of the ugly levels of **all the edges on this tree**, then what is the minimum possible ugly level of the tree, can you help him?
Input
T, represents there are
T test cases.
For each test case:
The first line contains two positive integers
n and
k, represent the tree has
n nodes,
k leaf nodes of which have already been setted their beautiful levels.
The next
(n−1) lines, each line contains two distinct positive integers
u and
v, repersent there is an undirected edge between node
u and node
v.
The next
k lines, each lines contains two positive integers
u and
w, repersent node
u is a leaf node, whose beautiful level is
w.
There is a blank between each integer with no other extra space in one line.
It's guaranteed that the input edges constitute a tree.
1≤T≤10,2≤n≤5⋅104,1≤k≤n,1≤u,v≤n,1≤w≤109
Output
For each test case, output in one line a non-negative integer, repersents the minimum possible ugly level of the tree.
Sample Input
2 3 2 1 2 1 3 2 4 3 9 6 2 1 2 1 3 1 4 2 5 2 6 3 6 5 9
Sample Output
Hint
