POJ1741(SummerTrainingDay08-G 树的点分治)

Tree

Time Limit: 1000MS		Memory Limit: 30000K
Total Submissions: 23380		Accepted: 7748

Description

Give a tree with n vertices,each edge has a length(positive integer less than 1001). 
Define dist(u,v)=The min distance between node u and v. 
Give an integer k,for every pair (u,v) of vertices is called valid if and only if dist(u,v) not exceed k. 
Write a program that will count how many pairs which are valid for a given tree. 

Input

The input contains several test cases. The first line of each test case contains two integers n, k. (n<=10000) The following n-1 lines each contains three integers u,v,l, which means there is an edge between node u and v of length l. 
The last test case is followed by two zeros. 

Output

For each test case output the answer on a single line.

Sample Input

5 4
1 2 3
1 3 1
1 4 2
3 5 1
0 0

Sample Output

8

Source

LouTiancheng@POJ

 

//2017-08-09
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <vector>

using namespace std;

const int N = 10010;
int n, k, MAX, root, cnt, answer;

//链式前向星
int head[N], tot;
struct Edge{
    int next, to, w;
}edge[N<<4];

void add_edge(int u, int v, int w){
    edge[tot].w = w;
    edge[tot].to = v;
    edge[tot].next = head[u];
    head[u] = tot++;
}

int size[N];//size[i]表示以i为根的子树的大小，包括i。
int maxson[N];//maxson[i]表示以i为根的子树的最大儿子的大小。
int dis[N];//dis[i]表示i到根的距离。
bool vis[N];//vis[i]用来标记i点是否被删除。

void init(int n){
    answer = 0;
    tot = 0;
    memset(vis, 0, sizeof(vis));
    memset(head, -1, sizeof(head));
}

//计算出子树的大小
void dfs_size(int u, int fa){
    size[u] = 1;
    maxson[u] = 0;
    for(int i = head[u]; ~i; i = edge[i].next){
        int v = edge[i].to;
        if(vis[v] || v == fa)continue;
        dfs_size(v, u);
        size[u] += size[v];
        if(size[v] > maxson[u])
              maxson[u] = size[v];
    }
}

//找子树的重心。最大子树最小的点即为树的重心。
void dfs_root(int r, int u, int fa){
    if(size[r] - size[u] > maxson[u])//size[r]-size[u]为u上面的树的尺寸
          maxson[u] = size[r] - size[u];
    if(maxson[u] < MAX){
        MAX = maxson[u];
        root = u;
    }
    for(int i = head[u]; ~i; i = edge[i].next){
        int v = edge[i].to;
        if(vis[v] || v == fa)continue;
        dfs_root(r, v, u);
    }
}

//计算出子树中每个点距离重心的距离
void dfs_dis(int u, int d, int fa){
    dis[cnt++] = d;
    for(int i = head[u]; ~i; i = edge[i].next){
        int v = edge[i].to;
        if(vis[v] || v == fa)continue;
        dfs_dis(v, d+edge[i].w, u);
    }
}

//计算出以u为根的子树中距离和小于k的点对数
int cal(int u, int d){
    int ans = 0;
    cnt = 0;
    dfs_dis(u, d, 0);
    sort(dis, dis+cnt);
    for(int i = 0, j = cnt-1; i < j; i++){
        while(dis[i]+dis[j] > k && i < j)//双指针
              j--;
        ans += j-i;
    }
    return ans;
}

//分治，找到树的重心，分为经过重心的点对和不经过重心的点对。
void solve(int u){
    MAX = n;
    dfs_size(u, 0);
    dfs_root(u, u, 0);
    answer += cal(root, 0);
    vis[root] = 1;
    for(int i = head[root]; ~i; i = edge[i].next){
        int v = edge[i].to;
        if(vis[v])continue;
        answer -= cal(v, edge[i].w);
        solve(v);
    }
}

int main()
{
    //freopen("dataIn.txt", "r", stdin);
    while(scanf("%d%d", &n, &k)!=EOF){
        if(!n && !k)break;
        int u, v, w;
        init(n);
        for(int i = 0; i < n-1; i++){
            scanf("%d%d%d", &u, &v, &w);
            add_edge(u, v, w);
            add_edge(v, u, w);
        }
        solve(1);
        printf("%d\n", answer);
    }

    return 0;
}