1439. [NOIP2013]貨車運輸

http://cogs.pro/cogs/problem/problem.php?pid=1439

★★☆ 輸入文件：truck.in 輸出文件：truck.out 簡單對比
時間限制：1 s 內存限制：128 MB

【題目描述】

【來源】

CCF全國信息學奧林匹克聯賽（NOIP2013）復賽Day1

#include<iostream>
#include<cstring>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
#define maxn 10010
#define maxm 50010
int n,m,f[maxn][20],sum[maxn][20],fa[maxn],num,p,dep[maxn],head[maxn]; 
bool vis[maxn];
 
struct node{
    int f,to,v;
}a[maxm];
struct Edge{
    int pre,v,to;
}e[maxm];
int cmp(node x,node y){return x.v>y.v;}
int find(int x){
    if(fa[x]==x)return fa[x];
    else return fa[x]=find(fa[x]);
}
int connect(int x,int y){
    int f1=find(x),f2=find(y);
    if(f1==f2)return 0;
    
 
else fa[f1]=f2;return 1;
}
void Insert(int from,int to,int v){
    e[++num].v=v;
    e[num].to=to;
    e[num].pre=head[from];
    head[from]=num;
}
void dfs(int now,int father,int deep){
    vis[now]=1;
    dep[now]=deep;
    for(int i=head[now];i;i=e[i].pre){
        int v=e[i].to;
        if(v==father)continue;
        f[v][0]=now;
        sum[v][0]=e[i].v;
        dfs(v,now,deep+1);
    }
}
int lca(int x,int y){
    if(x==y)return 0;
    int ans=0x3f3f3f3f;
    if(dep[x]<dep[y])swap(x,y);
    for(int i=18;i>=0;i--){
        if(dep[f[x][i]]>=dep[y]&&f[x][i]!=0)ans=min(ans,sum[x][i]),x=f[x][i];
    }
    if(x==y)return ans;
    for(int i=18;i>=0;i--){
        if(f[x][i]!=f[y][i]){
            ans=min(ans,sum[x][i]);
            ans=min(ans,sum[y][i]);
            x=f[x][i];y=f[y][i];
        }
    }
    ans=min(sum[x][0],ans);ans=min(sum[y][0],ans);
    return ans;
}
int main(){
    //freopen("Cola.txt","r",stdin);
    freopen("truck.in","r",stdin);
    freopen("truck.out","w",stdout);
    memset(sum,127/3,sizeof(sum));
    int x,y,z;
    scanf("%d%d",&n,&m);
    for(int i=1;i<=m;i++)scanf("%d%d%d",&a[i].f,&a[i].to,&a[i].v);
    for(int i=1;i<=n;i++)fa[i]=i;
    sort(a+1,a+m+1,cmp);int cnt=0;
    for(int i=1;i<=m;i++){
        if(connect(a[i].f,a[i].to)){
            Insert(a[i].f,a[i].to,a[i].v);
            Insert(a[i].to,a[i].f,a[i].v);
            cnt++;if(cnt==n-1)break;
        }
    }
    for(int i=1;i<=n;i++)if(!vis[i])dfs(i,0,1);
    for(int j=1;(1<<j)<=n;j++)
        for(int i=1;i<=n;i++)
            if(f[f[i][j-1]][j-1]!=0){
                f[i][j]=f[f[i][j-1]][j-1];
                sum[i][j]=min(sum[i][j-1],sum[f[i][j-1]][j-1]);
            }
    scanf("%d",&p);
    for(int i=1;i<=p;i++){
        scanf("%d%d",&x,&y);
        if(find(x)!=find(y)){printf("-1\n");continue;}
        printf("%d\n",lca(x,y));
    }
}

[NOIP2013]貨車運輸