【APIO2013】道路费用(TOLL)

先求一下原图的最小生成树,把不在最小生成树里的边全部删掉。

Mr.Greedy 的边会替换掉若干最小生成树上的边。

一个暴力做法是,先 (2 ^ k) 枚举哪些边一定在最小生成树中,把他们加入最小生成树。然后从小到达枚举原图上的边。如果能加入就直接加入,否则就会出现一个环,那么这个环上所有 Mr.Greedy 的边权值都不能大于这条边。这样的复杂度是 (O(2^kn))

考虑优化这个暴力。显然,有一些边是无论如何都会在最小生成树里的。将 Mr.Greedy 的 (k) 条边全部插入最小生成树中,然后加入原图的边。能在此时被加入的边,一定会存在于任何一棵最小生成树中。

那么就把这些边直接缩起来。我们得到的新图中就只有不超过 (k+1) 个点,(inom{k+1}{2}) 条边。

在新图上跑原来的暴力。现在我们只需考虑 (O(k^2)) 条原图的边对 (O(k)) 条 Mr.Greedy 的边的限制。

如果直接将一条路径的值取 Min,单次复杂度是 (O(k^3))。但是事实上,因为最小生成树从小到大加边,一条边最先被赋值的时候就应该被赋到了最小值,所以可以对每个点再维护第一个没有被赋值的祖先边。总复杂度就可以做到 (O(mlogm + 2^kk^2))

代码写的有点丑。

#pragma GCC optimize("2,Ofast,inline")
#include<bits/stdc++.h>
#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define LL long long
#define pii pair<int, int>
using namespace std;
const int N = 3e5 + 100;
const int inf = 0x3f3f3f3f;

template <typename T> T read(T &x) {
	int f = 0;
	register char c = getchar();
	while (c > '9' || c < '0') f |= (c == '-'), c = getchar();
	for (x = 0; c >= '0' && c <= '9'; c = getchar())
		x = (x << 3) + (x << 1) + (c ^ 48);
	if (f) x = -x;
	return x;
}

int n, m, k, tot;
int fa[N], peo[N], tag[N], dep[N], mn[N];
LL val[N], sum[N], siz[N];

struct Edge{
	int a, b, c;

	bool operator < (const Edge &e) const {
		return c < e.c;
	}
} e[N], gdy[N];

vector<int> V, G[N];

struct UFS {
	int top, u[N], v[N];
	int fa[N];
	LL siz[N];

	void init() {
		top = 0;
		for (int i = 1; i <= n; ++i) {
			fa[i] = i;
			siz[i] = peo[i];
		}
	}
	
	int find(int x) {
		return (fa[x] == x) ? x : find(fa[x]);
	}

	void merge(int x, int y) {
		x = find(x); y = find(y);
		if (siz[x] < siz[y]) swap(x, y);
		fa[y] = x; siz[x] += siz[y];
		++top; u[top] = x; v[top] = y;
	}

	void cancle() {
		int x = u[top], y = v[top--];
		fa[y] = y; siz[x] -= siz[y];
	}
} ufs, nmdp;

void dfs(int x, int f) {
	dep[x] = dep[f] + 1;
	sum[x] = val[x];
	mn[x] = inf;
	fa[x] = f;
	for (int i = 0; i < G[x].size(); ++i) {
		if (G[x][i] == f) continue;
		dfs(G[x][i], x);
		sum[x] += sum[G[x][i]];
	}
}

LL solve(int S) {
	for (int i = 0; i < V.size(); ++i) {
		G[V[i]].clear();
	}
	nmdp.top = 0;
	for (int i = 0; i < V.size(); ++i) {
		nmdp.fa[V[i]] = V[i];
	}
	int now = ufs.top, flag = 1, rt = ufs.find(1);
	for (int i = 0; i < k; ++i) {
		if (S >> i & 1) {
			int x = gdy[i + 1].a, y = gdy[i + 1].b;
			if (ufs.find(x) == ufs.find(y)) flag = 0;
			else {
				ufs.merge(x, y);
				G[x].pb(y);
				G[y].pb(x);
			}
		}
	}
	if (!flag) {
		while (ufs.top > now) {
			ufs.cancle();
		}
		return 0;
	}
	for (int i = 1; i <= tot; ++i) {
		int x = e[i].a;
		int y = e[i].b;
		if (ufs.find(x) == ufs.find(y)) {
			tag[i] = 1;
			continue;
		}
		ufs.merge(x, y);
		G[x].pb(y);
		G[y].pb(x);
	}
	dfs(rt, 0);
	for (int i = 1; i <= tot; ++i) {
		if (!tag[i]) continue;
		tag[i] = 0;
		int u = nmdp.find(e[i].a), v = nmdp.find(e[i].b), s = u;
		while (u != v) {
			if (dep[u] < dep[v]) swap(u, v);
			nmdp.merge(u, s);
			mn[u] = min(mn[u], e[i].c);
			u = fa[u];
		}
	}
	LL ans = 0;
	for (int i = 0; i < k; ++i) {
		if (S >> i & 1) {
			int u = gdy[i + 1].a, v = gdy[i + 1].b;
			if (dep[u] < dep[v]) swap(u, v);
			ans += 1LL * mn[u] * sum[u];
		}
	}
	while (ufs.top > now) {
		ufs.cancle();
	}
	return ans;
}

int main() {
	read(n); read(m); read(k);
	for (int i = 1; i <= m; ++i) {
		read(e[i].a); read(e[i].b); read(e[i].c);
	}
	for (int i = 1; i <= k; ++i) {
		read(gdy[i].a); read(gdy[i].b);
	}
	for (int i = 1; i <= n; ++i) read(peo[i]);
	sort(e + 1, e + m + 1);
	ufs.init();
	int cnt = 0;
	for (int i = 1; i <= m; ++i) {
		int x = ufs.find(e[i].a);
		int y = ufs.find(e[i].b);
		if (x == y) e[i].c = inf, ++cnt;
		else ufs.merge(x, y);
	}
	ufs.init();
	sort(e + 1, e + m + 1); m -= cnt;
	for (int i = 1; i <= k; ++i) {
		ufs.merge(gdy[i].a, gdy[i].b);
	}
	for (int i = 1; i <= m; ++i) {
		int x = ufs.find(e[i].a);
		int y = ufs.find(e[i].b);
		if (x == y) ++tot;
		else {
			ufs.merge(x, y);
			e[i].c = inf;
		}
	}
	ufs.init();
	for (int i = 1; i <= m; ++i) {
		if (e[i].c == inf) {
			ufs.merge(e[i].a, e[i].b);
		}
	}
	for (int i = 1; i <= n; ++i) {
		if (ufs.find(i) == i) {
			V.pb(i);
			val[i] = ufs.siz[i];
		}
	}
	for (int i = 1; i <= k; ++i) {
		gdy[i].a = ufs.find(gdy[i].a);
		gdy[i].b = ufs.find(gdy[i].b);
	}
	sort(e + 1, e + m + 1);
	for (int i = 1; i <= tot; ++i) {
		e[i].a = ufs.find(e[i].a);
		e[i].b = ufs.find(e[i].b);
	}
	LL ans = 0;
	for (int i = 1; i < (1 << k); ++i) {
		ans = max(ans, solve(i));
	}
	cout << ans << endl;
	return 0;
}
原文地址:https://www.cnblogs.com/Vexoben/p/11822271.html