HDU 2196 Computer（树的直径）

题意：给你一个树，求任意点对的最长距离

思路：首先任意选取一个点求一次DFS求出树上直径的一个端点，然后以该端点再一次DFS求出另一个树直径端点，然后再以该端点为起点DFS一波，每次都更新d数组。

原理：树上任意某个节点到树上任意节点的最远距离的端点一定会是树上直径的两个端点之一

#include <cstdio>
#include <queue>
#include <cstring>
#include <iostream>
#include <cstdlib>
#include <algorithm>
#include <vector>
#include <map>
#include <string>
#include <set>
#include <ctime>
#include <cmath>
#include <cctype>
using namespace std;
#define maxn 10010
#define LL long long
int cas=1,T;
struct Edge
{
int v,w;
Edge(int vv,int ww):v(vv),w(ww){}
};
int d[maxn];
int maxlen;
vector<vector<Edge> >e;
int End=0;
void dfs(int u,int fa,int len)
{
if (len>maxlen)
maxlen=len,End=u;
for (int i = 0;i<e[u].size();i++)
{
int v = e[u][i].v,w=e[u][i].w;
if (v==fa)
continue;
dfs(v,u,len+w);
d[v]=max(d[v],len+w);
}
}
int main()
{
int n;
while (scanf("%d",&n)!=EOF)
{
e.clear();
e.resize(n+2);
for (int i = 2;i<=n;i++)
{
int v,w;
scanf("%d%d",&v,&w);
e[i].push_back(Edge(v,w));
e[v].push_back(Edge(i,w));
}
memset(d,0,sizeof(d));
maxlen=0;
dfs(1,-1,0);
dfs(End,-1,0);
dfs(End,-1,0);
for (int i = 1;i<=n;i++)
printf("%d\n",d[i]);
}
//freopen("in","r",stdin);
//scanf("%d",&T);
//printf("time=%.3lf",(double)clock()/CLOCKS_PER_SEC);
return 0;
}

Description

A school bought the first computer some time ago(so this computer's id is 1). During the recent years the school bought N-1 new computers. Each new computer was connected to one of settled earlier. Managers of school are anxious about
slow functioning of the net and want to know the maximum distance Si for which i-th computer needs to send signal (i.e. length of cable to the most distant computer). You need to provide this information.



Hint: the example input is corresponding to this graph. And from the graph, you can see that the computer 4 is farthest one from 1, so S1 = 3. Computer 4 and 5 are the farthest ones from 2, so S2 = 2. Computer 5 is the farthest one from 3, so S3 = 3. we also
get S4 = 4, S5 = 4.

Input

Input file contains multiple test cases.In each case there is natural number N (N<=10000) in the first line, followed by (N-1) lines with descriptions of computers. i-th line contains two natural numbers - number of computer, to which
i-th computer is connected and length of cable used for connection. Total length of cable does not exceed 10^9. Numbers in lines of input are separated by a space.

Output

For each case output N lines. i-th line must contain number Si for i-th computer (1<=i<=N).

Sample Input

5
1 1
2 1
3 1
1 1

Sample Output

3
2
3
4
4