牛客 - Accumulation Degree （换根）

传送门：牛客 - Accumulation Degree

思路分析

换根的时候注意叶子节点的答案

样例输入

1
5
1 2 11
1 4 13
3 4 5
4 5 10

样例输出

26

AC代码

#include <functional>
#include <algorithm>
#include <iostream>
#include <cstring>
#include <cstdlib>
#include <iomanip>
#include <vector>
#include <string>
#include <cstdio>
#include <chrono>
#include <random>
#include <queue>
#include <stack>
#include <cmath>
#include <map>
#include <set>
#if __cplusplus >= 201103L
#include <unordered_map>
#include <unordered_set>
#endif
#define ls x<<1
#define rs x<<1|1
#define fi first
#define se second
#define ll long long
#define pb push_back
#define mp make_pair
#define fun function
#define sz(x) x.size()
#define lowbit(x) x&(-x)
#define all(x) x.begin(),x.end()
#define mem(a,b) memset(a,b,sizeof(a))
#define IOS ios_base::sync_with_stdio(0); cin.tie(0);cout.tie(0);
 
using namespace std;
 
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
 
const int INF = 0x3f3f3f3f;
const int N=2e5+10;
 
vector<pair<int,int>>e[N];
int dp[N],f[N],in[N];
 
void dfs1(int u,int fa) {
    for(auto x:e[u]) {
        int v=x.fi;
        int w=x.se;
        if(v==fa) continue;
        dfs1(v,u);
        if(in[v]==1) dp[u]+=w;
        else dp[u]+=min(dp[v],w);
    }
}
 
void dfs2(int u,int fa) {
    f[u]=dp[u];
    for(auto x:e[u]) {
        int v=x.fi;
        int w=x.se;
        if(v==fa) continue;
        int sdu=dp[u],sdv=dp[v];
        if(in[u]==1) dp[v]+=w;
        else dp[v]+=min(dp[u]-min(w,dp[v]),w);
        dfs2(v,u);
        dp[u]=sdu;
        dp[v]=sdv;
    }
}
 
int main() {
    IOS;
#ifdef xiaofan
    freopen("1.in","r",stdin);
    freopen("1.out","w",stdout);
#endif
 
    int t;
    cin>>t;
    while(t--) {
        for(int i=1; i<=200000; i++) e[i].clear();
        mem(dp,0);
        mem(f,0);
        mem(in,0);
        int n;
        cin>>n;
        for(int i=1; i<n; i++) {
            int u,v,w;
            cin>>u>>v>>w;
            e[u].push_back(mp(v,w));
            e[v].push_back(mp(u,w));
            in[u]++;
            in[v]++;
        }
        dfs1(1,1);
        dfs2(1,1);
        int ans=0;
        for(int i=1; i<=n; i++) ans=max(ans,f[i]);
        cout<<ans<<endl;
    }
 
 
 
 
    return 0;
}