（最小生成树 次小生成树）The Unique MST -- POJ -- 1679

链接：

http://poj.org/problem?id=1679

http://acm.hust.edu.cn/vjudge/contest/view.action?cid=82831#problem/K

Time Limit: 1000MS		Memory Limit: 10000K
Total Submissions: 24594		Accepted: 8751

Description

Given a connected undirected graph, tell if its minimum spanning tree is unique. 

Definition 1 (Spanning Tree): Consider a connected, undirected graph G = (V, E). A spanning tree of G is a subgraph of G, say T = (V', E'), with the following properties: 
1. V' = V. 
2. T is connected and acyclic. 

Definition 2 (Minimum Spanning Tree): Consider an edge-weighted, connected, undirected graph G = (V, E). The minimum spanning tree T = (V, E') of G is the spanning tree that has the smallest total cost. The total cost of T means the sum of the weights on all the edges in E'. 

Input

The first line contains a single integer t (1 <= t <= 20), the number of test cases. Each case represents a graph. It begins with a line containing two integers n and m (1 <= n <= 100), the number of nodes and edges. Each of the following m lines contains a triple (xi, yi, wi), indicating that xi and yi are connected by an edge with weight = wi. For any two nodes, there is at most one edge connecting them.

Output

For each input, if the MST is unique, print the total cost of it, or otherwise print the string 'Not Unique!'.

Sample Input

2
3 3
1 2 1
2 3 2
3 1 3
4 4
1 2 2
2 3 2
3 4 2
4 1 2

Sample Output

3
Not Unique!

代码：

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;

const int N = 110;
const int INF = 0x3f3f3f3f;

int J[N][N], dist[N], pre[N], Max[N][N], n, m;
int use[N][N];
bool vis[N];

int prim()
{
    int i, j, ans=0;
    memset(Max, 0, sizeof(Max));
    memset(use, 0, sizeof(use));
    memset(dist, 0, sizeof(dist));
    memset(vis, false, sizeof(vis));
    vis[1]=1;

    for(i=1; i<=n; i++)
    {
        dist[i]=J[1][i];
        pre[i]=1;
    }

    for(i=1; i<n; i++)
    {
        int index=1, MIN=INF;
        for(j=1; j<=n; j++)
        {
            if(!vis[j] && dist[j]<MIN)
            {
                MIN=dist[j];
                index=j;
            }
        }
        ans += MIN;
        vis[index]=1;
        use[index][pre[index]]=use[pre[index]][index]=1;

        for(j=1; j<=n; j++)
        {
            if(vis[j] && j!=index)
            {
                Max[index][j]=Max[j][index]=max(Max[j][pre[index]], dist[index]);
            }
            if(!vis[j] && dist[j]>J[index][j])
            {
                dist[j]=J[index][j];
                pre[j]=index;
            }
        }
    }
    return ans;
}

int cc(int s)
{
    int MIN=INF, i, j;
    for(i=1; i<=n; i++)
    for(j=i+1; j<=n; j++)
    {
       if(!use[i][j] && J[i][j]!=INF)
       {
           MIN = min(MIN, s-Max[i][j]+J[i][j]);
       }
    }
    return MIN;
}

int main ()
{
    int t;
    scanf("%d", &t);
    while(t--)
    {
        int i, j, a, b, c;
        scanf("%d%d", &n, &m);

        for(i=1; i<=n; i++)
        {
            J[i][i]=0;
            for(j=1; j<i; j++)
             J[i][j]=J[j][i]=INF;
        }


        for(i=1; i<=m; i++)
        {
            scanf("%d%d%d", &a, &b, &c);
            J[a][b]=J[b][a]=c;
        }

        int ans1=prim();
        int ans2=cc(ans1);
        if(ans1==ans2)
            printf("Not Unique!
");
        else
            printf("%d
", ans1);
    }
    return 0;
}