[luoguP1993] 小 K 的农场（差分约束 + spfa 判断负环）

传送门

差分约束系统。。找负环用spfa就行

——代码

 #include <cstdio>
 #include <cstring>
 #include <iostream>
 #define N 100001

 int n, m, cnt;
 int head[N], to[N << ], val[N << ], next[N << ], dis[N];
 bool vis[N];

 inline int read()
 {
     int x = , f = ;
     char ch = getchar();
     for(; !isdigit(ch); ch = getchar()) if(ch == '-') f = -;
     for(; isdigit(ch); ch = getchar()) x = (x << ) + (x << ) + ch - '';
     return x * f;
 }

 inline void add(int x, int y, int z)
 {
     to[cnt] = y;
     val[cnt] = z;
     next[cnt] = head[x];
     head[x] = cnt++;
 }

 inline bool spfa(int u)
 {
     int i, v;
     vis[u] = ;
     for(i = head[u]; i ^ -; i = next[i])
     {
         v = to[i];
         if(dis[v] > dis[u] + val[i])
         {
             dis[v] = dis[u] + val[i];
             if(vis[v]) return ;
             else if(!spfa(v)) return ;
         }
     }
     vis[u] = ;
     return ;
 }

 int main()
 {
     int i, j, opt, x, y, z;
     n = read();
     m = read();
     memset(head, -, sizeof(head));
     for(i = ; i <= m; i++)
     {
         opt = read();
         x = read();
         y = read();
         if(opt == ) add(x, y, ), add(y, x, );
         else
         {
             z = read();
             if(opt == ) add(x, y, -z);
             if(opt == ) add(y, x, z);
         }
     }
     memset(dis,  / , sizeof(dis));
     for(i = ; i <= n; i++) add(, i, );
     dis[] = ;
     spfa() ? puts("Yes") : puts("No");
     return ;
 }