[BZOJ 3365] Distance Statistics

[题目链接]

          https://www.lydsy.com/JudgeOnline/problem.php?id=3365

[算法] 

       点分治

[代码] 

         

#include <algorithm>  
#include <bitset>  
#include <cctype>  
#include <cerrno>  
#include <clocale>  
#include <cmath>  
#include <complex>  
#include <cstdio>  
#include <cstdlib>  
#include <cstring>  
#include <ctime>  
#include <deque>  
#include <exception>  
#include <fstream>  
#include <functional>  
#include <limits>  
#include <list>  
#include <map>  
#include <iomanip>  
#include <ios>  
#include <iosfwd>  
#include <iostream>  
#include <istream>  
#include <ostream>  
#include <queue>  
#include <set>  
#include <sstream>  
#include <stdexcept>  
#include <streambuf>  
#include <string>  
#include <utility>  
#include <vector>  
#include <cwchar>  
#include <cwctype>  
#include <stack>  
#include <limits.h> 
using namespace std;
#define MAXN 40010

int i,n,m,k,tot,len,ans,root,u,v,w;
int head[MAXN],weight[MAXN],size[MAXN],d[MAXN],dep[MAXN];
bool visited[MAXN];
char c;

struct Edge
{
    int to,w,nxt;
} e[MAXN<<1];

inline void addedge(int u,int v,int w)
{
    tot++;
    e[tot] = (Edge){v,w,head[u]};
    head[u] = tot;
}
inline void get_root(int u,int fa,int total)
{
    int i,v;
    size[u] = 1;
    weight[u] = 0;
    for (i = head[u]; i; i = e[i].nxt)
    {
        v = e[i].to;
        if (v != fa && !visited[v])
        {
            get_root(v,u,total);
            size[u] += size[v];
            weight[u] = max(weight[u],size[v]);
        }
    }    
    weight[u] = max(weight[u],total-size[u]);
    if (weight[u] < weight[root]) root = u;
}
inline void dfs(int u,int fa)
{
    int i,v,w;
    d[++len] = dep[u];
    for (i = head[u]; i; i = e[i].nxt)
    {
        v = e[i].to;
        w = e[i].w;
        if (!visited[v] && v != fa)
        {
            dep[v] = dep[u] + w;
            dfs(v,u);
        }    
    }
}
inline int calc(int u)
{
    int i,j;
    int ret = 0;
    len = 0;
    dfs(u,0);
    i = 1; j = len;
    sort(d+1,d+len+1);
    while (i < j)
    {
        if (d[i] + d[j] <= k) 
        {
            ret += j - i;
            i++;
        } else j--;
    }
    return ret;
}
inline void work(int u)
{
    int i,v,w;
    dep[u] = 0;
    visited[u] = true;
    ans += calc(u);
    for (i = head[u]; i; i = e[i].nxt)
    {
        v = e[i].to;
        w = e[i].w;
        if (!visited[v])
        {
            dep[v] = w;
            ans -= calc(v);
            root = 0;
            get_root(v,0,size[v]);
            work(root);
        }
    }
}

int main()
{
    
    scanf("%d%d",&n,&m);
    memset(visited,false,sizeof(visited));
    tot = 0;
    for (i = 1; i <= n; i++) head[i] = 0;
    for (i = 1; i < n; i++)
    {
        scanf("%d%d%d %c",&u,&v,&w,&c);
        addedge(u,v,w);
        addedge(v,u,w);
    }    
    scanf("%d",&k);
    size[0] = weight[0] = n;
    root = 0;
    get_root(1,0,0);
    ans = 0;
    work(root);
    printf("%d\n",ans);

    return 0;
}