HDU 4005 The war

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 const int maxn = 10010;
 4 const int INF = 0x3f3f3f3f;
 5 struct arc{
 6     int to,w,next;
 7     arc(int x = 0,int y = 0,int z = -1){
 8         to = x;
 9         w = y;
10         next = z;
11     }
12 }e[500000];
13 int hd[maxn],hd2[maxn],low[maxn],dfn[maxn],belong[maxn],clk,bcc,tot;
14 void add(int *head,int u,int v,int w){
15     e[tot] = arc(v,w,head[u]);
16     head[u] = tot++;
17 }
18 stack<int>stk;
19 void tarjan(int u,int fa){
20     dfn[u] = low[u] = ++clk;
21     stk.push(u);
22     bool flag = false;
23     for(int i = hd[u]; ~i; i = e[i].next){
24         if(e[i].to == fa && !flag){
25             flag = true;
26             continue;
27         }
28         if(!dfn[e[i].to]){
29             tarjan(e[i].to,u);
30             low[u] = min(low[u],low[e[i].to]);
31         }else low[u] = min(low[u],dfn[e[i].to]);
32     }
33     if(low[u] == dfn[u]){
34         int v;
35         ++bcc;
36         do{
37           v = stk.top();
38           stk.pop();
39           belong[v] = bcc;
40         }while(v != u);
41     }
42 }
43 
44 int ret = INF,dp[maxn];
45 void dfs(int u,int fa){
46     for(int i = hd2[u]; ~i; i = e[i].next){
47         if(e[i].to == fa) continue;
48         dfs(e[i].to,u);
49         dp[e[i].to] = min(dp[e[i].to],e[i].w);
50         if(dp[u] > dp[e[i].to]){
51             ret = min(ret,dp[u]);
52             dp[u] = dp[e[i].to];
53         }else ret = min(ret,dp[e[i].to]);
54     }
55 }
56 void init(){
57     for(int i = 0; i < maxn; ++i){
58         hd[i] = hd2[i] = -1;
59         dfn[i] = belong[i] = 0;
60         dp[i] = INF;
61     }
62     tot = bcc = clk = 0;
63     while(!stk.empty()) stk.pop();
64 }
65 int main(){
66     int n,m,u,v,w;
67     while(~scanf("%d%d",&n,&m)){
68         init();
69         for(int i = 0; i < m; ++i){
70             scanf("%d%d%d",&u,&v,&w);
71             add(hd,u,v,w);
72             add(hd,v,u,w);
73         }
74         for(int i = 1; i <= n; ++i)
75             if(!dfn[i]) tarjan(i,-1);
76         if(bcc == 1){
77             puts("-1");
78             continue;
79         }
80         int minW = INF,id = 0;
81         for(int tt = tot, i = 0; i < tt; i += 2){
82             if(belong[e[i].to] == belong[e[i^1].to]) continue;
83             add(hd2,belong[e[i].to],belong[e[i^1].to],e[i].w);
84             add(hd2,belong[e[i^1].to],belong[e[i].to],e[i].w);
85             if(e[i].w < minW) minW = e[id = i].w;
86         }
87         ret = INF;
88         dfs(belong[e[id].to],belong[e[id^1].to]);
89         dfs(belong[e[id^1].to],belong[e[id].to]);
90         printf("%d\n",ret == INF?-1:ret);
91     }
92     return 0;
93 }