[NOI2003]逃学的小孩

发现题目要求的相当于是在一颗树上面求\(dis(A,B)+min(dis(A,C),dis(B,C))\)

显然我们要最大化这个，前面的一定要是直径，后面那个直接随便乱求就行了。

/*
  mail: mleautomaton@foxmail.com
  author: MLEAutoMaton
  This Code is made by MLEAutoMaton
*/
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<math.h>
#include<algorithm>
#include<queue>
#include<set>
#include<map>
#include<iostream>
using namespace std;
#define ll long long
#define re register
#define int ll
#define file(a) freopen(a".in","r",stdin);freopen(a".out","w",stdout)
inline int gi(){
    int f=1,sum=0;char ch=getchar();
    while(ch>'9' || ch<'0'){if(ch=='-')f=-1;ch=getchar();}
    while(ch>='0' && ch<='9'){sum=(sum<<3)+(sum<<1)+ch-'0';ch=getchar();}
    return f*sum;
}
const int N=200010;
int n,m,front[N],cnt,dis1[N],dis2[N],vis[N];
struct node{int to,nxt,w;}e[N<<1];
void Add(int u,int v,int w){e[++cnt]=(node){v,front[u],w};front[u]=cnt;}
typedef pair<int,int> pii;
#define mp make_pair
priority_queue<pii,vector<pii>,greater<pii> >q;
void bfs1(int s){
    memset(dis1,63,sizeof(dis1));memset(vis,0,sizeof(vis));
    dis1[s]=0;
    q.push(mp(0,s));
    while(!q.empty()){
        int u=q.top().second;q.pop();
        if(vis[u])continue;vis[u]=1;
        for(int i=front[u];i;i=e[i].nxt){
            int v=e[i].to;
            if(dis1[v]>dis1[u]+e[i].w){
                dis1[v]=dis1[u]+e[i].w;
                q.push(mp(dis1[v],v));
            }
        }
    }
}
void bfs2(int s){
    memset(dis2,63,sizeof(dis2));memset(vis,0,sizeof(vis));
    dis2[s]=0;
    q.push(mp(0,s));
    while(!q.empty()){
        int u=q.top().second;q.pop();
        if(vis[u])continue;vis[u]=1;
        for(int i=front[u];i;i=e[i].nxt){
            int v=e[i].to;
            if(dis2[v]>dis2[u]+e[i].w){
                dis2[v]=dis2[u]+e[i].w;
                q.push(mp(dis2[v],v));
            }
        }
    }
}
signed main(){
    n=gi();m=gi();
    for(int i=1;i<=m;i++){
        int u=gi(),v=gi(),w=gi();
        Add(u,v,w);Add(v,u,w);
    }
    bfs1(1);
    int mx=0,id1=0,id2=0;
    for(int i=1;i<=n;i++)
        if(dis1[i]>mx){mx=dis1[i];id1=i;}
    bfs1(id1);mx=0;
    for(int i=1;i<=n;i++)
        if(dis1[i]>mx){mx=dis1[i];id2=i;}
    int ans=0;
    bfs2(id2);
    for(int i=1;i<=n;i++)
        ans=max(ans,min(dis1[i],dis2[i]));
    printf("%lld\n",mx+ans);
    return 0;
}

标签：dis1,dis2,小孩,int,逃学,vis,NOI2003,include,define
来源： https://www.cnblogs.com/mle-world/p/11146014.html