【换根DP+容斥】P3047 [USACO12FEB]Nearby Cows G

桃扇骨 2024-03-22 17:51 24阅读 0赞

[P3047 \[USACO12FEB\]Nearby Cows G - 洛谷 | 计算机科学教育新生态 (luogu.com.cn)][P3047 _USACO12FEB_Nearby Cows G - _ _ _ _luogu.com.cn]

题意：

![3a35b6178f1f45949716ab3f3c2821a7.png][]

思路：

做法就是换根

![994b1cc03c5e44688798d98c1d742f59.jpeg][]

预处理dp\[v\]\[j\]用普通的树形DP处理即可

注意：一开始预处理的dp\[v\]\[j\]指的是**在v的子树里**离v为j的权值和

Code：

#include <bits/stdc++.h>
    
    #define int long long
    #define max(a,b) (a>b?a:b)
    #define min(a,b) (a<b?a:b)
    
    using namespace std;
    
    const int mxn=3e6+10;
    const int mxe=3e5+10;
    const int mod=1e9+7;
    
    struct ty{
        int to,next;
    }edge[mxe<<2];
    
    int N,K,u,v,x,tot=0;
    int head[mxn],dp[mxn][22];
    
    void add(int u,int v){
        edge[tot].to=v;
        edge[tot].next=head[u];
        head[u]=tot++;
    }
    void G_init(){
        tot=0;
        for(int i=0;i<=N;i++){
            head[i]=-1;
        }
    }
    void dfs1(int u,int fa){
        for(int i=head[u];~i;i=edge[i].next){
            if(edge[i].to==fa) continue;
            dfs1(edge[i].to,u);
            for(int j=1;j<=K;j++){
                dp[u][j]+=dp[edge[i].to][j-1];
            }
        }
    }
    void dfs2(int u,int fa){
        for(int i=head[u];~i;i=edge[i].next){
            if(edge[i].to==fa) continue;
            for(int j=K;j>=2;j--){
                dp[edge[i].to][j]-=dp[edge[i].to][j-2];
            }
            for(int j=1;j<=K;j++){
                dp[edge[i].to][j]+=dp[u][j-1];
            }
            dfs2(edge[i].to,u);
        }
    }
    void solve(){
        cin>>N>>K;
        G_init();
        for(int i=1;i<=N-1;i++){
            cin>>u>>v;
            add(u,v);
            add(v,u);
        }
        for(int i=1;i<=N;i++){
            cin>>x;
            dp[i][0]=x;
        }
        dfs1(1,0);
        dfs2(1,0);
        for(int i=1;i<=N;i++){
            int ans=0;
            for(int j=0;j<=K;j++) ans+=dp[i][j];
            cout<<ans<<'\n';
        }
    }
    signed main(){
        ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
        int __=1;//cin>>__;
        while(__--)solve();return 0;
    }

2023.10.7 upd：

换根原来有通用写法，学会了，严格鸽是我爹

![a5fe0d1bf9ca414c9a43b33d5b18776f.png][]

#include <bits/stdc++.h>
    
    #define int long long
    
    using i64 = long long;
    
    #define lowbit(x) (x & (- x))
    
    constexpr int N = 1e5 + 10;
    constexpr int mod = 1e9 + 7;
    constexpr int Inf = 0x3f3f3f3f;
    constexpr double eps = 1e-10;
    
    int n, k;
    int c[N];
    int dp[N][33];
    std::vector<int> adj[N];
    
    void dfs1(int u, int fa) {
        for (auto v : adj[u]) {
            if (v == fa) continue;
            dfs1(v, u);
            for (int j = 1; j <= k; j ++) {
                dp[u][j] += dp[v][j - 1];
            }
        }
    }
    void dfs2(int u, int fa) {
        for (auto v : adj[u]) {
            if (v == fa) continue;
            std::vector<int> a(33);
            for (int j = 1; j <= k; j ++) {
                dp[u][j] -= dp[v][j - 1];
                a[j - 1] = dp[v][j - 1];
            }
            for (int j = 1; j <= k; j ++) {
                dp[v][j] += dp[u][j - 1];
            }
            dfs2(v, u);
            for (int j = 1; j <= k; j ++) {
                dp[u][j] += a[j - 1];
            }
        }
    }
    void solve() {
        std::cin >> n >> k;
        for (int i = 1; i <= n - 1; i ++) {
            int u, v;
            std::cin >> u >> v;
            adj[u].push_back(v);
            adj[v].push_back(u);
        }
        for (int i = 1; i <= n; i ++) std::cin >> c[i];
        for (int i = 1; i <= n; i ++) {
            dp[i][0] = c[i];
        }
        dfs1(1, 0);
        dfs2(1, 0);
        for (int i = 1; i <= n; i ++) {
            int ans = 0;
            for (int j = 0; j <= k; j ++) ans += dp[i][j];
            std::cout << ans << "\n";
        }
    }
    signed main() {
        std::ios::sync_with_stdio(false);
        std::cin.tie(nullptr);
    
        int t = 1;
        while(t --) {
            solve();
        }
        return 0;
    }

[P3047 _USACO12FEB_Nearby Cows G - _ _ _ _luogu.com.cn]: https://www.luogu.com.cn/problem/P3047
[3a35b6178f1f45949716ab3f3c2821a7.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/18/9afb524565654d4c94d55c44861008ea.png
[994b1cc03c5e44688798d98c1d742f59.jpeg]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/18/138296136fda48979352ad5e211a1fbe.png
[a5fe0d1bf9ca414c9a43b33d5b18776f.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/18/4d3706c26124400dbab5cc087d6dd158.png