HDU 4005 The war Tarjan+dp
The war
1 2 1
2 3 2
4 3
1 2 1
1 3 2
1 4 3
3
For the second sample input: our enemy may build line 2 to 3, 2 to 4,
3 to 4. If they build line 2 to 3, we will destroy line 1 to 4, cost 3. If they
build line 2 to 4, we will destroy line 1 to 3, cost 2. If they build line 3 to 4,
we will destroy line 1 to 2, cost 1. So, if we want to make sure that we can
destroy successfully, the minimum cost is 3.
#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<vector>
using namespace std;
#pragma comment(linker, "/STACK:102400000,102400000")
#define ls i<<1
#define rs ls | 1
#define pii pair<int,int>
#define MP make_pair
typedef long long LL;
const long long INF = 1e18;
const double Pi = acos(-1.0);
const int N = 1e4+, M = 1e5, mod = 1e9+, inf = 2e9; int ans,scc,t,top,tot,head[N],n,m,dp[N][],a[N],b[N],c[N];
struct edge{int id,to,next,value;}e[M]; void add(int u,int v,int w) {e[t].next = head[u];e[t].to=v;e[t].value=w;e[t].id=;head[u]=t++;} int dfn[N],q[N],inq[N],low[N],belong[N],hav[N]; vector<pair<int ,int > > G[N];
void init() {
for(int i = ; i <= n; ++i) dp[i][] = dp[i][] = inf;
for(int i = ; i <= n; ++i) G[i].clear();
memset(hav,,sizeof(hav));
memset(dfn,,sizeof(dfn));
memset(head,-,sizeof(head));
t = tot = top = scc = ;
}
void dfs(int u) {
low[u] = dfn[u] = ++tot;
q[++top] = u; inq[u] = ;
for(int i = head[u]; i!=-; i = e[i].next) {
int to = e[i].to;
if(e[i].id) continue;
e[i].id = e[i ^ ].id = ;
if(!dfn[to]) {
dfs(to);
low[u] = min(low[u],low[to]);
} else if(inq[to]) low[u] = min(low[u],dfn[to]);
}
if(low[u] == dfn[u]) {
scc++;
do{
inq[q[top]] = ;
belong[q[top]] = scc;
}while(u != q[top--]);
}
}
void dfs_ans(int u,int fa) {
if(u == -) return ;
int fi = ;
for(int i = ; i < G[u].size(); ++i) {
int to = G[u][i].first;
int value = G[u][i].second;
if(to == fa) continue;
dfs_ans(to,u);
if(!fi) {
dp[u][] = min(value,dp[to][]);
dp[u][] = dp[to][];
fi = ;
} else {
if(min(value,dp[to][]) < dp[u][]) dp[u][] = min(dp[u][],min(dp[to][],dp[u][])),dp[u][] = min(value,dp[to][]);
else dp[u][] = min(dp[u][],min(value,dp[to][]));
}
}
}
void Tarjan() {
int mi = inf, s = -, t = -;
for(int i = ; i <= n; ++i) if(!dfn[i]) dfs(i);
for(int i = ; i <= m; ++i) {
int fx = belong[a[i]];
int fy = belong[b[i]];
if(fx != fy) {
G[fx].push_back(MP(fy,c[i]));
G[fy].push_back(MP(fx,c[i]));
// cout<<fx<<" "<<fy<<endl;
if(c[i] < mi) {
s = fx,t = fy;
mi = c[i];
}
}
}
ans = inf;
dfs_ans(s,t);
dfs_ans(t,s);
if(s != - && t != -)ans = min(dp[s][],dp[t][]);
if(ans == inf) printf("%d\n",-);
else printf("%d\n",ans);
}
int main() {
while(~scanf("%d%d",&n,&m)) {
init();
for(int i = ; i <= m; ++i) {
scanf("%d%d%d",&a[i],&b[i],&c[i]);
add(a[i],b[i],c[i]);add(b[i],a[i],c[i]);
}
Tarjan();
}
return ;
}
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