【luogu P2984 [USACO10FEB]给巧克力Chocolate Giving】 题解

题目链接：https://www.luogu.org/problemnew/show/P2984

练习SPFA，把FJ当做起点，求出到所有牛的最短路，再把两个牛的相加。

 #include <cstdio>
 #include <queue>
 #include <iostream>
 #include <algorithm>
 #include <cstring>
 using namespace std;
 const int maxn = ;
 const int inf = 0x7fffffff;
 int n, m, b, dis[maxn];
 bool vis[maxn];
 struct edge{
     int len, to, next, from;
 }e[maxn];
 int head[maxn], cnt;
 queue<int> q;
 void add(int u, int v, int w)
 {
     e[++cnt].from = u;
     e[cnt].to = v;
     e[cnt].len = w;
     e[cnt].next = head[u];
     head[u] = cnt;
 }
 int SPFA()
 {
     while(!q.empty())
     {
         int now = q.front();
         q.pop();
         vis[now] = ;
         for(int i = head[now]; i ; i = e[i].next)
         {
             if(dis[e[i].to] > dis[now] + e[i].len)
             {
                 dis[e[i].to] = dis[now] + e[i].len;
                 if(!vis[e[i].to])
                 {
                     vis[e[i].to] = ;
                     q.push(e[i].to);
                 }
             }
         }
     }
 }
 int main()
 {
     scanf("%d%d%d",&n,&m,&b);
     for(int i = ; i <= m; i++)
     {
         int u,v,w;
         scanf("%d%d%d",&u,&v,&w);
         add(u,v,w);
         add(v,u,w);
     }
     for(int i = ; i <= n; i++)
     dis[i] = inf;
     dis[] = ;
     q.push();
     vis[] = ;
     SPFA();
     for(int i = ; i <= b; i++)
     {
         int p,q;
         scanf("%d%d",&p,&q);
         printf("%d\n",dis[p]+dis[q]);
     }
     return ;
 }