CF1254E Send Tree to Charlie

题意

讲不太清楚，看英文吧

cf

做法

在正式开始之前，我们先来玩一玩性质

首先考虑全\(0\)的情况，即本质不同的方案数

性质1：方案数并不为(n-1)!，即方案与结果不为双射

考虑一条边将树分为两个联通块，之间是互不影响的

结论1：方案数为\(\prod\limits (deg_i!)\)

考虑节点\(u\)的父节点与其父亲这条边已经选过

对于每种除\(u\)的儿子外的边固定时，\(u\)的儿子边的全排列决定了其对子树值的影响

ps：实际上这是无根树，但并不影响映射关系

性质2：若\(n\ge 2\)，\(a_i\neq i\)

结论2：若知晓所有限制，\(\sum\limits dis(i,a_i)=2(n-1)\)

具体考虑\(a_v=u\)的限制，有以下结论

• 存在相对顺序边集\((u,t_1)(t_1,t_2)...(t_k,v)\)
• \((u,t_1)\)为\(u\)的第一条边
• \((t_k,v)\)为\(v\)的最后一条边
• \((t_{i-1},t_{i})(t_i,t_{i+1})\)为\(t_i\)严格顺序的边

我们可以依此建立顺序，合法的方案，对于某个点，有以下限制

• 不可破坏简单顺序
• 不可生成环，即为一种非自然的复杂顺序
• 若有严格顺序从第一条边到最后一条边，中间不可漏边

方案数，对于无限制的边随便选择顺序

题外话

这题很妙，虽然容易理解，但需要想满条件还是需要时间的，由于没出现什么算法，强行归到拓扑排序上了

code(std)

#include <bits/stdc++.h>

using namespace std;

const int mod = 1e9 + 7;
const int maxn = 500005;

int n;
vector <int> adj[maxn];
int a[maxn];
int dad[maxn];
int h[maxn];
vector <pair <int, int> > conditions[maxn];
int nxt[maxn], prv[maxn], seen[maxn];

void no(int ncase) {
    cerr << ncase << endl;
    cout << 0 << endl;
    exit(0);
}

void dfs(int u) {
    for (auto v: adj[u]) if (v != dad[u]) {
        dad[v] = u;
        h[v] = h[u] + 1;
        dfs(v);
    }
}

int cnt; ///total distance, must not be more than 2n-2

void go(int u, int v) {
    if (u == v) no(0);
    vector <int> from_u, to_v;
    ///naive LCA works here as long as we exit upon finding a conflict
    from_u.push_back(n + 1); ///first edge (fake)
    to_v.push_back(n + 2); ///last edge (fake)
    while (h[u] > h[v]) {
        from_u.push_back(u);
        u = dad[u];
    }
    while (h[v] > h[u]) {
        to_v.push_back(v);
        v = dad[v];
    }
    while (u != v) {
        from_u.push_back(u);
        u = dad[u];
        to_v.push_back(v);
        v = dad[v];
    }
    from_u.push_back(u);
    from_u.insert(from_u.end(), to_v.rbegin(), to_v.rend());
    for (int i = 1; i + 1 < from_u.size(); ++i)
        conditions[from_u[i]].push_back({from_u[i-1], from_u[i+1]});
    cnt += from_u.size() - 3;
    if (cnt > 2 * n - 2) no(0); ///important break
}

int main(void) {
    ios_base::sync_with_stdio(0);
    cin.tie(NULL);
    cin >> n;
    for (int i = 1; i < n; ++i) {
        int u, v;
        cin >> u >> v;
        adj[u].push_back(v);
        adj[v].push_back(u);
    }
    for (int i = 1; i <= n; ++i) {
        adj[i].push_back(n + 1); ///first edge (fake)
        adj[i].push_back(n + 2); ///last edge (fake)
    }
    for (int i = 1; i <= n; ++i) cin >> a[i];

    if (n == 1) {
        cout << 1 << endl;
        exit(0);
    }

    dfs(1);
    for (int i = 1; i <= n; ++i) if (a[i] != 0) go(i, a[i]);

    ///answer 0 if:
    ///1. there are 2 or more incoming/outgoing
    ///conditions to/from an edge, or
    ///2. there is a cycle, or
    ///3. first (n+1) is connected to last (n+2),
    ///but does not contain all other edges.
    int ans = 1;
    for (int i = 1; i <= n; ++i) {
        ///check case 1
        for (auto edge: conditions[i]) {
            int u = edge.first, v = edge.second;
            if (nxt[u] && nxt[u] != v) no(1);
            if (prv[v] && prv[v] != u) no(1);
            nxt[u] = v;
            prv[v] = u;
        }
        ///check case 2
        for (auto u: adj[i]) if (!seen[u]) {
            seen[u] = 1;
            int cur = nxt[u];
            while (cur) {
                if (cur == u) no(2);
                if (seen[cur]) break;
                seen[cur] = 1;
                cur = nxt[cur];
            }
        }
        ///check case 3
        if (nxt[n+1]) {
            int cur = n + 1, all = 1;
            while (cur) {
                if (cur == n + 2) break;
                ++all;
                cur = nxt[cur];
            }
            if (cur == n + 2 && all < adj[i].size()) no(3);
        }
        ///all good - for now
        int free = 0;
        for (auto u: adj[i]) if (u <= n && !prv[u]) ///fake edges doesn't count
            ++free;
        if (prv[n+2]) --free; ///connected to last edge => not free
        for (int j = 1; j <= free; ++j) ans = 1ll * ans * j % mod;
        ///reset
        for (auto u: adj[i]) nxt[u] = prv[u] = seen[u] = 0;
    }

    ///no conflicts
    cout << ans << endl;
    return 0;
}