In the mathematical discipline of graph theory, the line graph of a simple undirected weighted graph G is another simple undirected weighted graph L(G) that represents the adjacency between every two edges in G
.
Precisely speaking, for an undirected weighted graph G
without loops or multiple edges, its line graph L(G)
is a graph such that:
- Each vertex of L(G)
represents an edge of G
- .
- Two vertices of L(G)
- , and the weight of such edge between this two vertices is the sum of their corresponding edges' weight.
A minimum spanning tree(MST) or minimum weight spanning tree is a subset of the edges of a connected, edge-weighted undirected graph that connects all the vertices together, without any cycles and with the minimum possible total edge weight. That is, it is a spanning tree whose sum of edge weights is as small as possible.
Given a tree G
, please write a program to find the minimum spanning tree of L(G).
InputThe first line of the input contains an integer T(1≤T≤1000)
, denoting the number of test cases.
In each test case, there is one integer n(2≤n≤100000)
in the first line, denoting the number of vertices of G.
For the next n−1
lines, each line contains three integers u,v,w(1≤u,v≤n,u≠v,1≤w≤109), denoting a bidirectional edge between vertex u and v with weight w.
It is guaranteed that ∑n≤106
.
OutputFor each test case, print a single line containing an integer, denoting the sum of all the edges' weight of MST(L(G))
.
Example Input2 4 1 2 1 2 3 2 3 4 3 4 1 2 1 1 3 1 1 4 1Output
8 4
题解:题目给出一张图,让我们将每个边看成一个”点“,两“点”之间的权值为两边权之和。让我们找到这个“点”组成的图(题目命名为”线图“)的最小生成树的权值和。
我们可以从每条边权(即每个点的出边)的贡献入手,首先一个点的出边必须连通,否则构不成最小生成树。
那么对于特定的一个点,首先将其所有出边全部的权值加一遍,然后将其最小的一个边权乘以(这一点的度degree-2)即保证了最优解。(其实这样就是连degree-1条边使得保证最优解)
对于每一个点都这样,跑一遍即可。
#include<iostream> #include<cstring> #include<string> #include<queue> #include<stack> #include<algorithm> #include<stdio.h> #include<map> #include<set> using namespace std; typedef long long ll; const int maxn=100010; struct node { int v,w; bool operator < (const node &r)const{ return w<r.w; } }; vector<node>G[maxn]; int main() { ios::sync_with_stdio(0); int T; cin>>T; while(T--){ int n; cin>>n; for(int i=1;i<=n;i++)G[i].clear(); for(int i=1;i<=n-1;i++){ int u,v,w; cin>>u>>v>>w; G[u].push_back((node){v,w}); G[v].push_back((node){u,w}); } ll ans=0; for(int i=1;i<=n;i++){ sort(G[i].begin(),G[i].end()); int minn=0x3f3f3f3f; int degree=G[i].size(); for(int j=0;j<degree;j++){ ans+=G[i][j].w; minn=min(minn,G[i][j].w); } ans+=(ll)minn*(degree-2);//当degree为1时与上面的ans相互消去 } cout<<ans<<endl; } return 0; }