ARC101E - Ribbons on Tree 树形DP

#include<bits/stdc++.h>#define pb push_backusing namespace std;const int N = 5000 + 10;
const int mod = 1e9 + 7;vector<int> G[N];
int f[N][N], g[N], tmp[N];
int n, size[N];void dfs(int u, int dad) {f[u][size[u] = 1] = 1;for(auto v : G[u]) if(v != dad) {dfs(v, u);for(int i = 1; i <= size[u] + size[v]; ++ i) tmp[i] = 0;for(int i = 1; i <= size[u]; ++ i)for(int j = 0; j <= size[v]; ++ j)(tmp[i + j] += 1ll * f[u][i] * f[v][j] % mod) %= mod;for(int i = 1; i <= size[u] + size[v]; ++ i)f[u][i] = tmp[i];size[u] += size[v];}for(int i = 2; i <= size[u]; i += 2)(f[u][0] += mod - 1ll * f[u][i] * g[i] % mod) %= mod;
}int main() {#ifdef ylsakioifreopen("e.in", "r", stdin);freopen("e.out", "w", stdout);
#endifint x, y; g[0] = 1;scanf("%d", &n);for(int i = 2; i <= n; i += 2)g[i] = 1ll * g[i - 2] * (i - 1) % mod;for(int i = 1; i < n; ++ i) {scanf("%d%d", &x, &y);G[x].pb(y), G[y].pb(x);}dfs(1, 0);printf("%d\n", mod - f[1][0]);return 0;
}

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ARC101E - Ribbons on Tree 树形DP

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