BZOJ 3365 Distance Statistics 点分治

这道题是一道点分治的题目，难度不大，可以拿来练手。

关键是对于找出来的重心的删除操作需要删掉这条边，这很重要。

还有每次找重心的时候，不但要考虑他的子节点的siz，还要考虑父节点的siz。

然后就A了。。。

每次点分治 分两种情况讨论一下就可以啦！

/w\...

#include <cstdio>
#include <algorithm>

using namespace std;

const int maxn = 56789;
const int INF = 1000000007;

int getint()
{
	int r = 0, k = 1;
	char c;
	for (c = getchar(); c < '0' || c > '9'; c = getchar() ) if (c == '-') k = -1;
	for (; '0' <= c && c <= '9'; c = getchar() ) r = r * 10 - '0' + c;
	return r * k;
}

int n, m, k;
int h[maxn], siz[maxn];

struct edge_type
{
	int v, next, w;
	bool baned;
} edge[maxn * 2]; int tote = 0;

void ins(int u, int v, int w)
{
	edge[++tote].v = v;
	edge[tote].next = h[u];
	edge[tote].w = w;
	edge[tote].baned = false;
	h[u] = tote;
}

int fgans, fgsiz;

void get_fgans(int now, int fa, int size) {
	int tmp = 1;
	siz[now] = 1;
	for (int i = h[now]; i; i = edge[i].next) {
		if (!edge[i].baned && fa != edge[i].v) {
			get_fgans(edge[i].v, now, size);
			siz[now] += siz[edge[i].v];
			if (tmp < siz[edge[i].v]) tmp = siz[edge[i].v];
		}
	}
	if (tmp < size - siz[now]) tmp = size - siz[now];
	if (tmp < fgsiz) {
		fgans = now;
		fgsiz = tmp;
	}
}
int find_gravity(int rt, int size) {
	fgsiz = INF;
	fgans = rt;
	get_fgans(rt, -1, size);
	return fgans;
}

int dis[maxn];
int zhan[maxn], zcnt;

void get_dis(int now, int nd, int fa) {
	dis[now] = nd;
	for (int i = h[now]; i; i = edge[i].next)
		if (!edge[i].baned && fa != edge[i].v)
			get_dis(edge[i].v, nd + edge[i].w, now);
}

void dfs(int now, int fa) {
	zhan[++zcnt] = dis[now];
	for (int i = h[now]; i; i = edge[i].next)
		if (!edge[i].baned && fa != edge[i].v)
			dfs(edge[i].v, now);
}

int calculate(int rt) {
	int ret = 0;
	zcnt = 0;
	dfs(rt, -1);
	sort(zhan + 1, zhan + zcnt + 1);
	for (int i = 1, j = zcnt; i <= zcnt; ++i) {
		for (; j && zhan[i] + zhan[j] > k; --j);
		ret += j;
	}
	return ret;
}

int Ans = 0;

void dfz(int now, int size)
{
	int rt = find_gravity(now, size);
	get_dis(rt, 0, -1);
	Ans += calculate(rt);
	int v;
	for (int i = h[rt]; i; i = edge[i].next)
		if (!edge[i].baned) {
			edge[i].baned = edge[((i-1)^1)+1].baned = true;
			Ans -= calculate(edge[i].v);
			dfz(edge[i].v, siz[edge[i].v]);
		}
}

int main()
{
	n = getint();
	m = getint();
	int u, v, w;
	for (int i = 1; i < n; ++i)
	{
		u = getint();
		v = getint();
		w = getint();
		ins (u, v, w);
		ins (v, u, w);
	}
	k = getint();
	dfz(1, n);
	printf("%d", (Ans - n >> 1));
	return 0;
}