UVA

和UVA - 1395有点相似，都是求最大边权和最小边权的差值。不过这里的边数要多得多，枚举边权会超时。
所以这里用的方法是最小生成树建完之后开始添边，添一条边就会形成一个环，找到环中最小的边删除。

#include<iostream>#include<string>#include<cstdio>#include<set>#include<stack>#include<list>#include<vector>#include<queue>#include<algorithm>#include<cstring>#include<cmath>#include<fstream>using namespace std;typedef long long ll;//ifstream cin;const int maxn = 350,maxe = maxn * maxn,INF = 0x3f3f3f3f;int n,m;struct node{    int u,v,w;    bool operator < (const node &a) const {        return w < a.w;    }}e[maxe];vector<pair<int,int>> r[maxn];int top[maxn];int find(int x){    return x == top[x] ? x : top[x] = find(top[x]);}bool vis[maxn * maxn];node path[maxn];int cnt;bool find_path(int fa,int u,int ed){//dfs找环，路径存在path数组内    if (u == ed) return true;    for(int i = 0;i < r[u].size();++i){        int v = r[u][i].first,w = r[u][i].second;        if (v == fa) continue;        path[cnt++] = {u,v,w};        if (find_path(u,v,ed)) return true;        cnt--;    }    return false;}inline void add(node &a){    int u = a.u,v = a.v,w = a.w;    r[u].push_back({v,w});    r[v].push_back({u,w});}inline void del(node &a){    int u = a.u,v = a.v;    for(vector<pair<int,int>>::iterator it = r[u].begin();it != r[u].end();++it){        if (it->first == v){            r[u].erase(it);            break;        }    }    for(vector<pair<int,int>>::iterator it = r[v].begin();it != r[v].end();++it){        if (it->first == u){            r[v].erase(it);            break;        }    }}void update(int u,int fa,int &mx,int &mi){//遍历树更新ans    for(int i = 0;i < r[u].size();++i){        int v = r[u][i].first,w = r[u][i].second;        if (v == fa) continue;        mi = min(mi,w);        mx = max(mx,w);        update(v,u,mx,mi);    }}void kruskal(){    memset(vis,0,sizeof vis);//最小生成树    for(int i = 0;i < n;++i) top[i] = i;    int sum = 1,ans = INF;    for(int k = 0;k < m;++k){        int u = e[k].u,v = e[k].v;        int fu = find(u),fv = find(v);        if (fu != fv){            top[fu] = fv;            add(e[k]);            vis[k] = 1;            sum++;        }        if (sum == n){            ans = e[k].w - e[0].w;            break;        }    }    for(int k = 0;k < m;++k){//加边更新ans        if (vis[k]) continue;        int u = e[k].u,v = e[k].v;        int mx = 0,mi = INF;        cnt = 0;        find_path(u,u,v);        sort(path,path + cnt);        del(path[0]);        add(e[k]);        update(0,0,mx,mi);        ans = min(ans,mx - mi);    }    printf("%d\n",ans);}void init(){    int a,b,c;    memset(e,0,sizeof e);    for(int i = 0;i < n;++i) r[i].clear();    for(int i = 0;i < m;++i){        cin >> a >> b >> c;        e[i] = {a,b,c};    }    sort(e,e + m);    kruskal();}int main(){    //cin.open("in.txt");    while(cin >> n && n){        cin >> m;        init();    }}