BZOJ 4078: [Wf2014]Metal Processing Plant

#include<algorithm>
#include<iostream>
#include<cstring>
#include<cstdio>
#include<ctime>
//by NeighThorn
#define inf 0x3f3f3f3f
using namespace std;
 
const int maxn=400+5,maxm=200000+5;
 
int id;
int lala,fa[maxn],co[maxn];
int C,tim,top,mp[maxn],dfn[maxn],low[maxn],stk[maxm],instk[maxn];
int n,s1,s2,ans,cnt,len,w[maxn][maxn],hd[maxn],to[maxm],nxt[maxm];
 
struct M{
     
    int x,y,v;
     
    inline M(){};
     
    M(int a,int b,int c){
        x=a,y=b,v=c;
    }
     
    friend bool operator < (M a,M b){
        if(a.v!=b.v)
			return a.v>b.v;
		if(a.x!=b.x)
			return a.x>b.x;
		return a.y>b.y;
    }
     
}e[maxm];
 
inline int read(void){
    char ch=getchar();int x=0;
    while(!(ch>=‘0‘&&ch<=‘9‘)) ch=getchar();
    while(ch>=‘0‘&&ch<=‘9‘) x=x*10+ch-‘0‘,ch=getchar();
    return x;   
}
 
inline void add(int x,int y){
    to[cnt]=y;nxt[cnt]=hd[x];hd[x]=cnt++;
}
 
inline void tarjan(int x){
    low[x]=dfn[x]=++tim;stk[++top]=x;instk[x]=1;
    for(int i=hd[x];i!=-1;i=nxt[i]){
        if(!dfn[to[i]])
            tarjan(to[i]),low[x]=min(low[x],low[to[i]]);
        else if(instk[to[i]])
            low[x]=min(low[x],dfn[to[i]]);
    }
    if(dfn[x]==low[x]){
        C++;int tmp;
        do{
            tmp=stk[top--],instk[tmp]=0;mp[tmp]=C;
        }while(tmp!=x);
    }
}
 
inline bool check(void){
    cnt=C=tim=top=0;
    memset(mp,0,sizeof(mp));
    memset(hd,-1,sizeof(hd));
    memset(dfn,0,sizeof(dfn));
    memset(low,0,sizeof(low));
    memset(instk,0,sizeof(instk));
    for(int i=1;i<n;i++)
        for(int j=i+1;j<=n;j++){
            if(w[i][j]>s1)
                add(i<<1,j<<1|1),add(j<<1,i<<1|1);
            if(w[i][j]>s2)
                add(i<<1|1,j<<1),add(j<<1|1,i<<1);
        }
    for(int i=2;i<=(n<<1|1);i++)
        if(!dfn[i]) tarjan(i);
    for(int i=1;i<=n;i++)
        if(mp[i<<1]==mp[i<<1|1])
            return false;
    return true;
}
 
inline int find(int x){
    if(fa[x]==x)
        return x;
    int fx=find(fa[x]);
    co[x]^=co[fa[x]];
    return fa[x]=fx;
}

inline int calc(int x){
	int l=0,r=x,res=-1;
	while(l<=r){
        int mid=(l+r)>>1;s2=mid;
        if(check())
            r=mid-1,res=mid;
        else
            l=mid+1;
    }
    return res;
}
 
signed main(void){
    n=read();ans=inf;
    if(n<=2) return puts("0"),0;
    for(int i=1;i<n;i++)
        for(int j=i+1;j<=n;j++)
            w[i][j]=w[j][i]=read(),e[++lala]=M(i,j,w[i][j]);
    sort(e+1,e+lala+1);for(int i=1;i<=n;i++) fa[i]=i;
    for(int i=1,x,y,fx,fy,res;i<=lala;i++){
        s1=e[i].v;x=e[i].x,y=e[i].y,fx=find(x),fy=find(y);
        if(fx!=fy){
        	res=calc(s1);
        	if(res!=-1) ans=min(ans,s1+res);
        	co[fx]=co[x]^co[y]^1;fa[fx]=fy;
        }
        else if(co[x]==co[y]){
        	res=calc(s1);
        	if(res!=-1) ans=min(ans,res+s1);
			break;
        }
    }
    printf("%d\n",ans);
    return 0;
}