Connections between cities

http://acm.hdu.edu.cn/showproblem.php?pid=2874

题解：LCA

参考文章： https://blog.csdn.net/weixin_43272781/article/details/88797088

/*
*@Author:   STZG
*@Language: C++
*/
#include <bits/stdc++.h>
#include<iostream>
#include<algorithm>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<string>
#include<vector>
#include<bitset>
#include<queue>
#include<deque>
#include<stack>
#include<cmath>
#include<list>
#include<map>
#include<set>
//#define DEBUG
#define RI register int
#define endl "\n"
using namespace std;
typedef long long ll;
//typedef __int128 lll;
const int N=100000+10;
const int M=100000+10;
const int MOD=1e9+7;
const double PI = acos(-1.0);
const double EXP = 1E-8;
const int INF = 0x3f3f3f3f;
int t,n,m,k,p,l,r,u,v,c;
int ans,cnt,flag,temp,sum,num;
int pre[N];
bool vis[N];
int dep[N];
int dp[N][21];
int dist[N];
int que[N];
int first[N];
char str;
struct node{
    int v,w;
}x;
struct query{
    int e,id;
}y;
vector<node>G[N];
int find(int x){return pre[x]==x?x:pre[x]=find(pre[x]);}
void marge(int u,int v){
    int tu=find(u);
    int tv=find(v);
    if(tu!=tv){
        pre[tv]=tu;
    }
}
void dfs(int u,int deep){
    vis[u]=1;
    que[++num]=u;//num在此处相当于dfs_clock
    first[u]=num;//表示第一次走的编号
    dep[num]=deep;
    for(int i=0,j=G[u].size();i<j;i++){
            int v=G[u][i].v;
            int w=G[u][i].w;
        if(!vis[v]){
            dist[v]=dist[u]+w;
            dfs(v,deep+1);
            que[++num]=u;
            dep[num]=deep;
        }
    }
}
void ST(int n){
    for(int i=1;i<=n;i++){
        dp[i][0]=i;
    }
    for(int i=0;(1<<i)<n;i++){
        for(int j=1;j+(1<<i)<n;j++){
            int a=dp[j][i],b=dp[j+(1<<i)][i];
            dp[j][i+1]=dep[a]<dep[b]?a:b;
        }
    }
}
int RMQ(int l,int r){
    k=log2(r-l+1);
    int a=dp[l][k],b=dp[r-(1<<k)+1][k];
    return dep[a]<dep[b]?a:b;
}
int LCA(int u,int v){
    int x = first[u] , y = first[v];
    if(x > y) swap(x,y);
    int res = RMQ(x,y);
    return que[res];
}
void init(){
    for(int i=1;i<=n;i++){
       G[i].clear();
       pre[i]=i;
    }
    memset(vis,0,sizeof(vis));
    memset(dep,0,sizeof(dep));
    memset(dist,0,sizeof(dist));
    memset(dp,0,sizeof(dp));
    memset(que,0,sizeof(que));
    cnt=0,num=0;
}
int main()
{
#ifdef DEBUG
	freopen("input.in", "r", stdin);
	//freopen("output.out", "w", stdout);
#endif
    //ios::sync_with_stdio(false);
    //cin.tie(0);
    //cout.tie(0);
    //scanf("%d",&t);
    while(~scanf("%d%d%d",&n,&m,&c)){
        init();
        for(int i=1;i<=m;i++){
            scanf("%d%d%d",&u,&v,&k);
            x.v=v;
            x.w=k;
            G[u].push_back(x);
            x.v=u;
            G[v].push_back(x);
            marge(u,v);
        }
        for(int i=1;i<=n;i++){
            if(pre[i]==i){
                dist[i]=0;
                dfs(i,1);
            }
        }
        ST(2*n-1);
        for(int i=1;i<=c;i++){
            scanf("%d%d",&u,&v);
            if(find(u)==find(v)){
                int lca=LCA(u,v);
                cout<<dist[u]+dist[v]-2*dist[lca]<<endl;
            }else{
                cout<<"Not connected"<<endl;
            }
        }
    }

#ifdef DEBUG
	printf("Time cost : %lf s\n",(double)clock()/CLOCKS_PER_SEC);
#endif
    //cout << "Hello world!" << endl;
    return 0;
}