USACO07FEB银牛派对

Posted 2020-11-22 phemiku

【题意】n个点m条有向边，求单源最短路最长往返距离

【解题】建两个图

跑两遍spfa

不过有很多细节需要注意

#include<bits/stdc++.h>
const int N = 1005;
struct node{
    int v,w;
};
std::vector<node>g[3][N];

std::queue<int>q;
int n,k,m,x,y,z,hs,vis[N],f[3][N],ans = -0x3f3f;

void spfa(int k){
    
    memset(vis,0,sizeof vis);
    vis[hs] = 1;
    f[k][hs] = 0;
    q.push(hs);
    while(!q.empty()){
        int u = q.front();
        q.pop();
        vis[u] = 0; 
        //std::cout << "DEBUG!"<<g[k][u].size();
        for(int i = 0;i < g[k][u].size();++i){
            int v = g[k][u][i].v, w = g[k][u][i].w;
            
            if(f[k][v] > f[k][u] + w){
                f[k][v] = f[k][u] + w;
                if(!vis[v]){
                    vis[v] = 1;
                    q.push(v);
                }
            }
        }
    }
}


int main(){
  scanf("%d%d%d",&n,&m,&hs);
  memset(f,0x3f3f3f,sizeof(f));
  for(register int i = 1; i <= m; ++i){
      scanf("%d%d%d",&x,&y,&z);
      g[1][x].push_back((node){y,z});
      g[2][y].push_back((node){x,z});
  } 
  spfa(1);
  spfa(2);
  
  for(register int i = 1; i <= n; ++i){
       if(i == hs)continue;
       ans = std::max(ans,f[1][i] + f[2][i]);
  }
  printf("%d",ans);
  return 0;    
}

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