[LA 3887] Slim Span

3887 - Slim Span
Time limit: 3.000 seconds

Given an undirected weighted graph G <tex2html_verbatim_mark>, you should find one of spanning trees specified as follows.

The graph G <tex2html_verbatim_mark>is an ordered pair (V, E) <tex2html_verbatim_mark>, where V <tex2html_verbatim_mark>is a set of vertices {v₁, v₂,..., v_n} <tex2html_verbatim_mark>and E <tex2html_verbatim_mark>is a set of undirected edges {e₁, e₂,..., e_m} <tex2html_verbatim_mark>. Each edge e  E <tex2html_verbatim_mark>has its weight w(e) <tex2html_verbatim_mark>.

A spanning tree T <tex2html_verbatim_mark>is a tree (a connected subgraph without cycles) which connects all the n <tex2html_verbatim_mark>vertices with n - 1 <tex2html_verbatim_mark>edges. The slimness of a spanning tree T <tex2html_verbatim_mark>is defined as the difference between the largest weight and the smallest weight among the n - 1 <tex2html_verbatim_mark>edges of T <tex2html_verbatim_mark>.

<tex2html_verbatim_mark>

For example, a graph G <tex2html_verbatim_mark>in Figure 5(a) has four vertices {v₁, v₂, v₃, v₄} <tex2html_verbatim_mark>and five undirected edges {e₁, e₂,e₃, e₄, e₅} <tex2html_verbatim_mark>. The weights of the edges are w(e₁) = 3 <tex2html_verbatim_mark>, w(e₂) = 5 <tex2html_verbatim_mark>, w(e₃) = 6 <tex2html_verbatim_mark>, w(e₄) = 6 <tex2html_verbatim_mark>, w(e₅) = 7 <tex2html_verbatim_mark>as shown in Figure 5(b).

=6in <tex2html_verbatim_mark>

There are several spanning trees for G <tex2html_verbatim_mark>. Four of them are depicted in Figure 6(a)∼(d). The spanning tree T_a<tex2html_verbatim_mark>in Figure 6(a) has three edges whose weights are 3, 6 and 7. The largest weight is 7 and the smallest weight is 3 so that the slimness of the tree T_a <tex2html_verbatim_mark>is 4. The slimnesses of spanning trees T_b <tex2html_verbatim_mark>, T_c <tex2html_verbatim_mark>and T_d <tex2html_verbatim_mark>shown in Figure 6(b), (c) and (d) are 3, 2 and 1, respectively. You can easily see the slimness of any other spanning tree is greater than or equal to 1, thus the spanning tree T_d <tex2html_verbatim_mark>in Figure 6(d) is one of the slimmest spanning trees whose slimness is 1.

Your job is to write a program that computes the smallest slimness.

Input 

The input consists of multiple datasets, followed by a line containing two zeros separated by a space. Each dataset has the following format.

n <tex2html_verbatim_mark>m <tex2html_verbatim_mark>
a₁ <tex2html_verbatim_mark>b₁ <tex2html_verbatim_mark>w₁ <tex2html_verbatim_mark>
 <tex2html_verbatim_mark>
a_m <tex2html_verbatim_mark>b_m <tex2html_verbatim_mark>w_m <tex2html_verbatim_mark>

Every input item in a dataset is a non-negative integer. Items in a line are separated by a space.

n <tex2html_verbatim_mark>is the number of the vertices and m <tex2html_verbatim_mark>the number of the edges. You can assume 2n100 <tex2html_verbatim_mark>and 0mn(n - 1)/2<tex2html_verbatim_mark>. a_k <tex2html_verbatim_mark>and b_k <tex2html_verbatim_mark>(k = 1,..., m) <tex2html_verbatim_mark>are positive integers less than or equal to n <tex2html_verbatim_mark>, which represent the two verticesv_ak <tex2html_verbatim_mark>and v_bk <tex2html_verbatim_mark>connected by the k <tex2html_verbatim_mark>-th edge e_k <tex2html_verbatim_mark>. w_k <tex2html_verbatim_mark>is a positive integer less than or equal to 10000, which indicates the weight of e_k <tex2html_verbatim_mark>. You can assume that the graph G = (V, E) <tex2html_verbatim_mark>is simple, that is, there are no self-loops (that connect the same vertex) nor parallel edges (that are two or more edges whose both ends are the same two vertices).

Output 

For each dataset, if the graph has spanning trees, the smallest slimness among them should be printed. Otherwise, `-1' should be printed. An output should not contain extra characters.

Sample Input 

4 5 
1 2 3
1 3 5
1 4 6
2 4 6
3 4 7
4 6 
1 2 10 
1 3 100 
1 4 90 
2 3 20 
2 4 80 
3 4 40 
2 1 
1 2 1
3 0 
3 1 
1 2 1
3 3 
1 2 2
2 3 5 
1 3 6 
5 10 
1 2 110 
1 3 120 
1 4 130 
1 5 120 
2 3 110 
2 4 120 
2 5 130 
3 4 120 
3 5 110 
4 5 120 
5 10 
1 2 9384 
1 3 887 
1 4 2778 
1 5 6916 
2 3 7794 
2 4 8336 
2 5 5387 
3 4 493 
3 5 6650 
4 5 1422 
5 8 
1 2 1 
2 3 100 
3 4 100 
4 5 100 
1 5 50 
2 5 50 
3 5 50 
4 1 150 
0 0

Sample Output 

1 
20 
0 
-1 
-1 
1 
0 
1686 
50

枚举最小边，求得MST

#include <iostream>
#include <cstring>
#include <algorithm>
#include <cstdio>
using namespace std;
#define INF 0x3f3f3f3f
#define N 110
#define M 100010

struct Edge
{
    int u,v,w;
    bool operator <(const Edge &t)const
    {
        return w<t.w;
    }
}edge[M];

int n,m;
int f[N];

void init()
{
    for(int i=1;i<=n;i++) f[i]=i;
}
int Find(int x)
{
    if(x!=f[x]) f[x]=Find(f[x]);
    return f[x];
}
bool UN(int x,int y)
{
    x=Find(x);
    y=Find(y);
    if(x==y) return 0;
    f[x]=y;
    return 1;
}
int kruskal(int s)
{
    init();
    int ret;
    for(int i=s;i<=m;i++)
    {
        if(UN(edge[i].u,edge[i].v)) ret=edge[i].w;
    }
    int cnt=0;
    for(int i=1;i<=n;i++) if(f[i]==i) cnt++;
    if(cnt>1) return -1;
    return ret;
}
int main()
{
    int ans;
    while(scanf("%d%d",&n,&m),n||m)
    {
        ans=INF;
        for(int i=1;i<=m;i++) scanf("%d%d%d",&edge[i].u,&edge[i].v,&edge[i].w);
        sort(edge+1,edge+m+1);
        for(int i=1;i<=m;i++)
        {
            int t=kruskal(i);
            if(t==-1) break;
            ans=min(ans,t-edge[i].w);
        }
        if(ans==INF) ans=-1;
        printf("%d
",ans);
    }
    return 0;
}