【luogu P1821 [USACO07FEB]银牛派对Silver Cow Party】 题解

题目链接：https://www.luogu.org/problemnew/show/P1821

反向多存一个图，暴力跑两遍

 #include <cstdio>
 #include <iostream>
 #include <algorithm>
 #include <queue>
 #include <cstring>
 using namespace std;
 const int maxn = ;
 const int inf = 0x7fffffff;
 int n, m, x, dis1[maxn], dis2[maxn];
 bool vis1[maxn] = {}, vis2[maxn] = {};
 struct edge{
     int len, to, next;
 }e1[maxn], e2[maxn];
 int head1[maxn], head2[maxn], cnt1, cnt2;
 void SPFA1()
 {
     queue<int> q1;
     vis1[x] = ;
     dis1[x] = ;
     q1.push(x);
     while(!q1.empty())
     {
         int now1 = q1.front();
         q1.pop();
         vis1[now1] = ;
         for(int i = head1[now1]; i!= ; i = e1[i].next)
         {
             if(dis1[e1[i].to] > dis1[now1]+e1[i].len)
             {
                 dis1[e1[i].to] = dis1[now1]+e1[i].len;
                 if(!vis1[e1[i].to])
                 {
                     vis1[e1[i].to] = ;
                     q1.push(e1[i].to);
                 }
             }
         }
     }
 }
 void SPFA2()
 {
     queue<int> q2;
     vis2[x] = ;
     dis2[x] = ;
     q2.push(x);
     while(!q2.empty())
     {
         int now2 = q2.front();
         q2.pop();
         vis2[now2] = ;
         for(int i = head2[now2]; i ; i = e2[i].next)
         {
             if(dis2[e2[i].to] > dis2[now2]+e2[i].len)
             {
                 dis2[e2[i].to] = dis2[now2]+e2[i].len;
                 if(!vis2[e2[i].to])
                 {
                     vis2[e2[i].to] = ;
                     q2.push(e2[i].to);
                 }
             }
         }
     }
 }
 int main()
 {
     cin>>n>>m>>x;
     for(int i = ; i <= n; i++)
     {
         dis2[i] = inf;
         dis1[i] = inf;
     }
     for(int i = ; i <= m; i++)
     {
         int u,v,w;
         cin>>u>>v>>w;
         e1[i].to = v;
         e1[i].len = w;
         e1[i].next = head1[u];
         head1[u] = i;

         e2[i].to = u;
         e2[i].len = w;
         e2[i].next = head2[v];
         head2[v] = i;
     }
     SPFA1();
     SPFA2();
     int ans = ;
     for(int i = ; i <= n; i++)
     {
         ans = max(dis1[i]+dis2[i],ans);
     }
     cout<<ans;
     return ;
 }

Ctrl+C Ctrl+V 真毒瘤，弄得我12分反好几次，真是老年OI选手