[DK] 化学竞赛的大奖

https://www.luogu.org/problemnew/show/T16502

无向图  缩点  树的直径  到直径两个端点的距离的较大值

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <cstring>

using namespace std;
const int N = 2e5 + ;

#define yxy getchar()

int n, m, now = , now_2 = , Tarjan_tim, Top, Bel_tim, Root, Max_d;
int head[N], head_2[N], Stack[N], Dfn[N], Low[N], Bel[N], dis[N], dist[N];
bool vis[N];
struct Node {int u, v, w, nxt;} E[N << ];
struct Node_2 {int u, v, w, nxt;} G[N << ];

inline int read(){
    int x = ; char c = yxy;
    while(c < '' || c > '') c = yxy;
    while(c >= '' && c <= '') x = x *  + c - '', c = yxy;
    return x;
}

void add_E(int u, int v, int w){
    E[now].v = v; E[now].w = w; E[now].nxt = head[u]; head[u] = now ++;
}

void Tarjan(int u, int fa){
    Dfn[u] = Low[u] = ++ Tarjan_tim;
    vis[u] = ;
    Stack[++ Top] = u;
    for(int i = head[u]; ~ i; i = E[i].nxt) {
        int v = E[i].v;
        if(!Dfn[v]){
            Tarjan(v, u);
            Low[u] = min(Low[u], Low[v]);
        }
        else if(vis[v] && v != fa) Low[u] = min(Low[u], Low[v]);
    }
    if(Dfn[u] == Low[u]){
        ++ Bel_tim;
        Bel[u] = Bel_tim; vis[u] = ;
        while(Stack[Top] != u){
            Bel[Stack[Top]] = Bel_tim;
            vis[Stack[Top]] = ;
            Top --;
        } Top --;
    }
}

void add_G(int u, int v, int w){
    G[now_2].v = v; G[now_2].w = w; G[now_2].nxt = head_2[u]; head_2[u] = now_2 ++;
}

void Build_G(){
    for(int i = ; i <= n; i ++) head_2[i] = -;
    for(int u = ; u <= n; u ++)
        for(int i = head[u]; ~ i; i = E[i].nxt)
            if(Bel[u] != Bel[E[i].v])
                add_G(Bel[u], Bel[E[i].v], E[i].w);
}

void dfs_first(int u, int fa){
    for(int i = head_2[u]; ~ i; i = G[i].nxt){
        int v = G[i].v;
        if(v == fa) continue ;
        dis[v] = dis[u] + G[i].w;
        if(dis[v] > Max_d) Max_d = dis[v], Root = v;
        dfs_first(v, u);
    }
}

void dfs_second(int u, int fa){
    for(int i = head_2[u]; ~ i; i = G[i].nxt){
        int v = G[i].v;
        if(v == fa) continue ;
        dist[v] = dist[u] + G[i].w;
        dfs_second(v, u);
    }
}

void Get_Answer(){
    dfs_first(, -);
    memset(dis, , sizeof dis);
    Max_d = ;
    dfs_first(Root, -);
    Max_d = ;
    dfs_second(Root, -);
    for(int i = ; i <= n; i ++) printf("%d\n", max(dis[Bel[i]], dist[Bel[i]]));
}

int main()
{
    n = read(); m = read();
    for(int i = ; i <= n; i ++) head[i] = -;
    for(int i = ; i <= m; i ++){
        int u = read(), v = read(), w = read();
        add_E(u, v, w); add_E(v, u, w);
    }
    for(int i = ; i <= n; i ++) if(!Dfn[i]) Tarjan(i, );
    Build_G();
    Get_Answer();
    return ;
}