【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,\(\binom{k+1}{2}\) 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;
}

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转载自www.cnblogs.com/Vexoben/p/11822271.html