题目链接

https://www.luogu.org/problemnew/show/P1967

分析

NOIp的一道裸题,直接在最大生成树上剖分取最小值一下就完事了，非常好写,常数也比较小，然而题解里有许多我没见过的船新操作,先挖个坑等有时间再看

注意

• 树链剖分又在第一遍挂了,忘了写top[now]=t;

• 注意题目说明并没有保证是联通的!!!然后成功被Hack了.这真的要警惕,指不定哪天毒瘤出题人就在这里把你正解卡成60(flag++)

代码

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <cctype>
#include <vector>
#include <queue>
#define ll long ong
#define ri register int
#define ull unsigned long long
using std::vector;
using std::swap;
using std::min;
using std::max;
using std::sort;
template <class T>inline void read(T &x){x=0;int ne=0;char c;while(!isdigit(c=getchar()))ne=c=='-';x=c-48;while(isdigit(c=getchar()))x=(x<<3)+(x<<1)+c-48;x=ne?-x:x;return ;
}
const int maxn=10005;
const int maxm=50005;
const int inf=0x7fffffff;
int n,m;
struct Edge{int x,y,c;Edge(int _x,int _y,int _c){x=_x,y=_y,c=_c;}Edge(){x=y=c=0;}bool operator <(const Edge &b)const {return c>b.c;}
}edge[maxm];
struct Dat{int ver,dis;Dat(int x,int y){ver=x,dis=y;}Dat(){;}
};
vector<Dat>g[maxn];
int pa[maxn];
int get(int x){if(pa[x]!=x)pa[x]=get(pa[x]);//return pa[x]==x?pa[x]:pa[x]=get(pa[x]);return pa[x];
}
inline void kruskal(){int cnt=0,x,y,xx,yy,c;sort(edge+1,edge+1+m);for(ri i=1;i<=n;i++)pa[i]=i;for(ri i=1;i<=m;i++){//printf("%d\n",edge[i].c);int x=edge[i].x,y=edge[i].y;xx=get(x),yy=get(y);if(xx==yy)continue;c=edge[i].c;//printf("%d %d %d\n",x,y,c);g[x].push_back(Dat(y,c));g[y].push_back(Dat(x,c));pa[xx]=yy;cnt++;if(cnt==n-1)break;}return ;
}
int dep[maxn],son[maxn],top[maxn],size[maxn],dfn[maxn],fa[maxn],rnk[maxn],tot=0;
int w[maxn];
void dfs_1(int now){int v;size[now]=1;for(ri i=0;i<g[now].size();i++){v=g[now][i].ver;if(v==fa[now])continue;fa[v]=now,dep[v]=dep[now]+1;w[v]=g[now][i].dis;dfs_1(v);size[now]+=size[v];if(!son[now]||size[v]>size[son[now]])son[now]=v;}return ;
}
void dfs_2(int now,int t){int v;dfn[now]=++tot,rnk[tot]=now,top[now]=t;if(!son[now])return ;dfs_2(son[now],t);for(ri i=0;i<g[now].size();i++){v=g[now][i].ver;if(v==fa[now]|v==son[now])continue;dfs_2(v,v);}return ;
}
int mi[maxn<<2];
void build(int now,int l,int r){if(l==r){mi[now]=w[rnk[l]];return ;}int mid=(l+r)>>1;build(now<<1,l,mid);build(now<<1|1,mid+1,r);mi[now]=min(mi[now<<1],mi[now<<1|1]);return ;
}
int L,R;
int query(int now,int l,int r){if(L<=l&&r<=R){return mi[now];}int ans=inf,mid=(l+r)>>1;if(L<=mid)ans=min(ans,query(now<<1,l,mid));if(mid<R)ans=min(ans,query(now<<1|1,mid+1,r));return ans;
}
inline int query_path(int x,int y){int ans=inf;while(top[x]!=top[y]){if(dep[top[x]]<dep[top[y]])swap(x,y);L=dfn[top[x]],R=dfn[x];ans=min(ans,query(1,1,n));x=fa[top[x]];}if(dfn[x]>dfn[y])swap(x,y);L=dfn[x]+1,R=dfn[y];if(L>R)return ans;ans=min(ans,query(1,1,n));return ans;
}
int main(){int q,x,y,z;read(n),read(m);for(ri i=1;i<=m;i++){read(x),read(y),read(z);edge[i]=Edge(x,y,z);}kruskal();for(ri i=1;i<=n;i++){//不一定联通if(!dfn[i]){dep[i]=1,fa[i]=0;dfs_1(i);dfs_2(i,i);}}build(1,1,n);read(q);while(q--){read(x),read(y);int tmp=query_path(x,y);if(!tmp)puts("-1");else printf("%d\n",tmp);}return 0;
}