POJ 1987 Distance Statistics 树的分治

题目：http://poj.org/problem?id=1987

题意：和POJ 1741基本一样

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;

const int N = 50010;
struct edge
{
    int to, val, next;
}g[N*2];
int head[N], siz[N], sonsiz[N], dis[N];
bool vis[N];
int n, m, k, cnt, num, res, tmp, root;
void add_edge(int v, int u, int val)
{
    g[cnt].to = u;
    g[cnt].val = val;
    g[cnt].next = head[v];
    head[v] = cnt++;
}
void dfssiz(int v, int fa)
{
    siz[v] = 1, sonsiz[v] = 0;
    for(int i = head[v]; i != -1; i = g[i].next)
    {
        int u = g[i].to;
        if(u != fa && !vis[u])
        {
            dfssiz(u, v);
            siz[v] += siz[u];
            sonsiz[v] = max(sonsiz[v], siz[u]);
        }
    }
}
void dfsroot(int r, int v, int fa)
{
    sonsiz[v] = max(sonsiz[v], siz[r] - siz[v]);
    if(sonsiz[v] < tmp) tmp = sonsiz[v], root = v;
    for(int i = head[v]; i != -1; i = g[i].next)
    {
        int u = g[i].to;
        if(u != fa && !vis[u]) dfsroot(r, u, v);
    }
}
void dfsdis(int v, int d, int fa)
{
    dis[num++] = d;
    for(int i = head[v]; i != -1; i = g[i].next)
    {
        int u = g[i].to;
        if(u != fa && !vis[u]) dfsdis(u, d + g[i].val, v);
    }
}
int calc(int v, int d)
{
    int ans = 0;
    num = 0;
    dfsdis(v, d, 0);
    sort(dis, dis + num);
    int i = 0, j = num - 1;
    while(i < j)
    {
        while(dis[i] + dis[j] > k && i < j) j--;
        ans += j - i;
        i++;
    }
    return ans;
}
void dfs(int v)
{
    tmp = n;
    dfssiz(v, 0);
    dfsroot(v, v, 0);
    res += calc(root, 0);
    vis[root] = true;
    for(int i = head[root]; i != -1; i = g[i].next)
    {
        int u = g[i].to;
        if(! vis[u])
        {
            res -= calc(u, g[i].val);
            dfs(u);
        }
    }
}
int main()
{
    int a, b, c;
    char ch;
    scanf("%d%d", &n, &m);
    cnt = res = 0;
    memset(head, -1, sizeof head);
    memset(vis, 0, sizeof vis);
    for(int i = 0; i < n - 1; i++)
    {
        scanf("%d%d%d %c", &a, &b, &c, &ch);
        add_edge(a, b, c);
        add_edge(b, a, c);
    }
    scanf("%d", &k);
    dfs(1);
    printf("%d\n", res);

    return 0;
}