先做一遍prim，求出最小生成树的value，和路径。每次去掉一条路径上的边，再做最小生成树，若出现新的value和第一次的value相等，则次小生成树不唯一。否则，取枚举过程中最小的一个value即可。
#include < stdio.h >
#include < string .h >
#define  N 110
#define  M 1<<27
int  g[N][N], pre[N],low[N];
 bool  visit[N];
 struct  Now
{
     int  x, y, z;
}now[N], cur;
 int  prim( int  n){
     int  i,j,k, ans = 0 ;

     for (i = 1 ; i <= n; i ++ ){
        low[i] = g[ 1 ][i];
        pre[i] = 1 ;
        visit[i] = false ;
    }
    visit[ 1 ] = true ;
    k = 1 ;
     for (i = 2 ; i <= n; i ++ ){
         int  mn =- 1 , p;
         for (j = 1 ; j <= n; j ++ )
             if ( ! visit[j]  &&  (mn ==- 1   ||  low[j] < mn)){
                mn = low[j];
                p = j;
            }
             if (mn ==- 1 )  break ;
            ans += mn;
            k = p;
            visit[k] = true ;
             for (j = 1 ; j <= n; j ++ ){
                 if ( ! visit[j]  &&  low[j] > g[k][j]){
                    low[j] = g[k][j];
                    pre[j] = k;
                }
            }

        }
     return  ans;
}
 int  main()
{
     int  i,j,n,m,ca,x,y,z;
    scanf( " %d " ,  & ca);
     while (ca -- ){
        scanf( " %d %d " ,  & n,  & m);
         for (i = 0  ; i <= n ; i ++ )
             for (j = 0 ; j <= n; j ++ )
                g[i][j] = M;
         for (i = 0 ; i < m; i ++ ){
            scanf( " %d %d %d " ,  & x,  & y,  & z);
            g[x][y] = g[y][x] = z;
        }
         int  value = prim(n);
        j = 0 ;
         for (i = 2 ; i <= n; i ++ ){
            now[j].x = i;
            now[j ++ ].y = pre[i];
        }
         int  t = 0 ;
         for (i = 0 ; i < n - 1 ; i ++ ){
             int  a, b, aa, bb;
            a = now[i].x;
            b = now[i].y;
            now[i].z = g[a][b];
            g[a][b] = g[b][a] = M;
             if (i > 0 ){
                aa = now[i - 1 ].x;
                bb = now[i - 1 ].y;
                g[aa][bb] = g[bb][aa] = now[i - 1 ].z;
            }
             int  tmp = prim(n);
             if (tmp == value){
                t = 1 ;
                 break ;
            }
        }
         if (t)
            printf( " Not Unique!\n " );
         else
            printf( " %d\n " , value);
    }
     return   0 ;
}