update

src/P3379/P3379.cpp

@@ -48,6 +48,7 @@ ll lca(ll u,ll v){
         }
     }
     return fa[u][0];
+    
 }
 
 int main(){

src/P5021/P5021.cpp

@@ -0,0 +1,66 @@
+#include <bits/stdc++.h>
+
+using ll = int64_t;
+using std::cin,std::cout;
+
+const ll maxn=5e4+5, inf=(ll(1)<<60);
+ll n,m,mins{inf},maxs,g[maxn],f[maxn];
+std::vector<std::pair<ll,ll>> adj[maxn];
+
+void dfs(ll fth,ll u,const ll &num){
+    std::vector<ll> vec;
+    for(auto [v,w]:adj[u]){
+        if(fth==v)continue;
+        dfs(u,v,num);
+        f[u]+=f[v];
+        if(ll ff=g[v]+w;ff>=num){
+            ++f[u];
+        }else{
+            vec.emplace_back(ff);
+        }
+    }
+    std::sort(vec.begin(),vec.end());
+    ll l{0},r{ll(vec.size())-1};
+    while(l<r){
+        if(vec[l]+vec[r]>=num){
+            f[u]++;
+            l++,r--;
+        }else{
+            l++;
+        }
+    }
+    if(l==r){
+        g[u]=vec[r];
+    }
+}
+
+bool check(ll num){
+    std::fill(f,f+1+n,0);
+    std::fill(g,g+1+n,0);
+    dfs(0,1,num);
+    return f[1]>=m;
+}
+
+int main(){
+    cin>>n>>m;
+    for(ll i{1};i<n;i++){
+        ll u,v,w;
+        cin>>u>>v>>w;
+        mins=std::min(mins,w);
+        maxs+=w;
+        adj[u].emplace_back(v,w);
+        adj[v].emplace_back(u,w);
+    }
+
+    ll l{mins},r{maxs},ans{l},mid;
+    while(l<=r){
+        mid=(l+r)/2;
+        if(check(mid)){
+            ans=mid;
+            l=mid+1;
+        }else{
+            r=mid-1;
+        }
+    }
+    cout<<ans<<'\n';
+}