【BZOJ 1468】Tree 点分治

点分治$O(nlogn)$

坚持到月考结束后新校就剩下我一个OIer，其他人早已停课了，老师估计懒得为我一个人开机房门，让我跟班主任说了一声，今晚就回到了老校，开始了自己都没有想到会来的这么早的停课生活。

所以先水一道点分治

#include<cstdio>
#include<algorithm>
#define read(x) x=getint()
#define N 40003
#define max(a,b) (a)>(b)?(a):(b)
using namespace std;
inline int getint() {
	char c; int fh=1, k=0;
	for( ; c < '0' || c > '9'; c=getchar()) if ( c == '-') fh = -1;
	for( ; c >= '0' && c <= '9'; c=getchar()) k = k * 10 + c - '0';
	return k * fh;
}
struct node {
	int nxt, to, w;
} E[N << 1];
bool vis[N];
int root, rtm = N, n, K, cnt = 0, sz[N], di[N], tb[N], tn, ans = 0, point[N];
inline void ins( int x, int y, int z) {cnt++; E[cnt].nxt = point[x]; E[cnt].to = y; E[cnt].w = z; point[x] = cnt;}
inline void fdrt( int x, int fa, int s) {
	sz[x] = 1;
	int ma=0;
	for( int tmp = point[x]; tmp; tmp = E[tmp].nxt)
		if ( !vis[E[tmp].to] && E[tmp].to != fa) {
			fdrt( E[tmp].to, x, s);
			sz[x] += sz[E[tmp].to];
			ma = max( ma, sz[E[tmp].to]);
		}
	ma = max( ma, s - ma);
	if ( ma < rtm) {
		rtm = ma;
		root = x;
	}
}
inline void mktb( int x, int fa) {
	tb[++tn] = di[x];
	for( int tmp = point[x]; tmp; tmp = E[tmp].nxt)
		if ( !vis[E[tmp].to] && E[tmp].to != fa) {
			di[E[tmp].to] = di[x] + E[tmp].w;
			mktb( E[tmp].to, x);
		}
}
inline int work( int x, int la) {
	di[x] = la;
	tn = 0;
	mktb( x, -1);
	sort( tb + 1, tb + tn + 1);
	int head = 1, tail = tn, an = 0;
	while ( head < tail) {
		while ( head < tail && tb[head] + tb[tail] > K)
			--tail;
		an += tail - head;
		++head;
	}
	return an;
}
inline void dfs( int x, int s) {
	vis[x] = 1;
	ans += work( x, 0);
	for( int tmp = point[x]; tmp; tmp = E[tmp].nxt)
		if ( !vis[E[tmp].to]) {
			ans -= work( E[tmp].to, E[tmp].w);
			rtm = N;
			int ss = sz[x] > sz[E[tmp].to] ? sz[E[tmp].to] : s - sz[x];
			fdrt( E[tmp].to, x, ss);
			dfs( root, ss);
		}
}
int main() {
	read(n);
	int u, v, e;
	for( int i = 1; i < n; ++i) {
		read(u); read(v); read(e);
		ins( u, v, e);
		ins( v, u, e);
	}
	read(K);
	fdrt( 1, -1, n);
	dfs( root, n);
	printf( "%d
", ans);
	return 0;
}

第一次交我忘了sort了，，，