Slim Span
题目：
Description
Given an undirected weighted graph G, you should find one of spanning trees specified as follows.
The graph G is an ordered pair (V, E), where V is a set of vertices {v1, v2, …, vn} and E is a set of undirected edges {e1, e2, …, em}. Each edge e ∈ E has its weight w(e).
A spanning tree T is a tree (a connected subgraph without cycles) which connects all the n vertices with n − 1 edges. The slimness of a spanning tree T is defined as the difference between the largest weight and the smallest weight among the n − 1 edges of T.
Figure 5: A graph G and the weights of the edges
For example, a graph G in Figure 5(a) has four vertices {v1, v2, v3, v4} and five undirected edges {e1, e2, e3, e4, e5}. The weights of the edges are w(e1) = 3, w(e2) = 5, w(e3) = 6, w(e4) = 6, w(e5) = 7 as shown in Figure 5(b).
Figure 6: Examples of the spanning trees of G
There are several spanning trees for G. Four of them are depicted in Figure 6(a)~(d). The spanning tree Ta 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 Ta is 4. The slimnesses of spanning trees Tb, Tc and Td 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 Td 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 | m | |
| a1 | b1 | w1 |
| ⋮ | ||
| am | bm | wm |
Every input item in a dataset is a non-negative integer. Items in a line are separated by a space. n is the number of the vertices and m the number of the edges. You can assume 2 ≤ n ≤ 100 and 0 ≤ m ≤ n(n− 1)/2. ak and bk (k = 1, …, m) are positive integers less than or equal to n, which represent the two vertices vak and vbk connected by the kth edge ek. wk is a positive integer less than or equal to 10000, which indicates the weight of ek. You can assume that the graph G = (V, E) 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
这题其实就是求出所有的生成树中每一颗生成树的最大权值减最小边的最小值！我们可以用kruskal和枚举把其所有的生成树都弄出来！然后在对其最小和最大边和当前最小差值进行判断！小酒赋值，大就不用管！代码如下！有一些解释：
#include"stdio.h"
#include"stdlib.h"
#define inf 0x7fffffff
#include"string.h"
typedef struct
{
int weight;
int s,v;
}EE;
EE edg[5001];
int set[101];
int cmp(const void *a,const void *b)
{
EE *c=(EE *)a;
EE *d=(EE *)b;
return c->weight-d->weight;
}
int Find(int x)
{
while(set[x]>=0)
x=set[x];
return x;
}
int main()
{
int n,m,ps,pv,i,j,tmp,ans;
while(scanf("%d%d",&n,&m)!=EOF)
{
if(n==0&&m==0)
break;
for(i = 0;i < m;i ++)
scanf("%d%d%d",&edg[i].s,&edg[i].v,&edg[i].weight);
qsort(edg ,m,sizeof(edg[0]),cmp);
ans=inf;
for(i=0;i<m;i ++)
{
tmp = 0;
memset(set,-1,sizeof(set));
for(j = i;j < m ;j ++)//kruskal；
{
ps = Find(edg[j].s);
pv = Find(edg[j].v);
if(pv!= ps)
{
tmp ++;
set[pv] = ps;
if(tmp == n - 1)
break;
}
}
if(j < m)//当前又生成树
{
if(edg[j].weight-edg[i].weight<ans)//最大权值边和最小边比较！因为最小的肯定是deg[i]的；
ans=edg[j].weight-edg[i].weight;
}
else//当前没有生成树
break;
}
if(ans==inf)//如果没有生成树，ans的值没变；
printf("-1\n");
else
printf("%d\n",ans);
}
return 0;
}
