luogu P1768 天路

嘟嘟嘟

01分数规划之最优比率环。

主要是发一下基于dfs的spfa。跑的贼快，原来总用时2000多ms还TLE了两个点，改成dfs后总用时直降43ms！

 #include<cstdio>
 #include<iostream>
 #include<algorithm>
 #include<cstring>
 #include<cmath>
 #include<cstdlib>
 #include<cctype>
 #include<stack>
 #include<queue>
 #include<vector>
 using namespace std;
 #define enter puts("")
 #define space putchar(' ')
 #define Mem(a, x) memset(a, x, sizeof(a))
 #define rg register
 typedef long long ll;
 typedef double db;
 const int INF = 0x3f3f3f3f;
 const db eps = 1e-;
 const int maxn = 7e3 + ;
 const int maxe = 4e4 + ;
 inline ll read()
 {
   ll ans = ;
   char ch = getchar(), las = ' ';
   while(!isdigit(ch)) las = ch, ch = getchar();
   while(isdigit(ch)) ans = (ans << ) + (ans << ) + ch - '', ch = getchar();
   if(las == '-') ans = -ans;
   return ans;
 }
 inline void write(ll x)
 {
   if(x < ) putchar('-'), x = -x;
   if(x >= ) write(x / );
   putchar(x %  + '');
 }

 int n, m;
 struct Edge
 {
   int nxt, to, v, p;
 }e[maxe];
 int head[maxn], ecnt = -;
 void addEdge(int x, int y, int v, int p)
 {
   e[++ecnt] = (Edge){head[x], y, v, p};
   head[x] = ecnt;
 }

 bool in[maxn];
 db dis[maxn];
 bool judge(db x, int now)
 {
   in[now] = ;
   for(int i = head[now]; i != -; i = e[i].nxt)
     {
       if(dis[e[i].to] > x * (db)e[i].p - (db)e[i].v + dis[now])
     {
       if(in[e[i].to]) return ;
       dis[e[i].to] = x * (db)e[i].p - (db)e[i].v + dis[now];
       in[e[i].to] = ;
       if(judge(x, e[i].to)) return ;
     }
     }
   in[now] = ;
   return ;
 }

 int main()
 {
   Mem(head, -);
   n = read(); m = read();
   for(int i = ; i <= m; ++i)
     {
       int x = read(), y = read(), v = read(), p = read();
       addEdge(x, y, v, p);
     }
   for(int i = ; i <= n; ++i) addEdge(, i, , ); //搞一个超级源点
   db L = , R = ;
   while(R - L > eps)
     {
       db mid = (L + R) / ;
       Mem(dis, 0x3f); Mem(in, ); dis[] = ;
       if(judge(mid, )) L = mid;
       else R = mid - eps;
     }
   if(L < eps) write(-), enter;
   else printf("%.1lf\n", L);
   return ;
 }