BZOJ1738 [Usaco2005 mar]Ombrophobic Bovines 发抖的牛

先预处理出来每个点对之间的最短距离

然后二分答案，网络流判断是否可行就好了恩

 /**************************************************************
     Problem: 1738
     User: rausen
     Language: C++
     Result: Accepted
     Time:404 ms
     Memory:9788 kb
 ****************************************************************/

 #include <cstdio>
 #include <cstring>
 #include <algorithm>

 using namespace std;
 typedef long long ll;
 const int N = ;
 const int M = N * N << ;
 const int inf_int = 1e9;
 const ll inf_ll = (ll) 1e18;

 inline int read();

 struct edge {
     int next, to, f;
     edge(int _n = , int _t = , int _f = ) : next(_n), to(_t), f(_f) {}
 } e[M];

 int n, m, S, T;
 int a[N], b[N], sum;
 ll mp[N][N];
 int first[N], tot;
 int d[N];

 inline void Add_Edges(int x, int y, int z) {
     e[++tot] = edge(first[x], y, z), first[x] = tot;
     e[++tot] = edge(first[y], x), first[y] = tot;
 }
 #define y e[x].to
 #define p q[l]
 bool bfs() {
     static int l, r, x, q[N];
     memset(d, -, sizeof(d));
     d[q[] = S] = ;
     for (l = r = ; l != r + ; ++l)
         for (x = first[p]; x; x = e[x].next)
             if (!~d[y] && e[x].f) {
                 d[q[++r] = y] = d[p] + ;
                 if (y == T) return ;
             }
     return ;
 }
 #undef p

 int dfs(int p, int lim) {
   if (p == T || !lim) return lim;
   int x, tmp, rest = lim;
   for (x = first[p]; x && rest; x = e[x].next)
     if (d[y] == d[p] +  && ((tmp = min(e[x].f, rest)) > )) {
       rest -= (tmp = dfs(y, tmp));
       e[x].f -= tmp, e[x ^ ].f += tmp;
       if (!rest) return lim;
     }
   if (rest) d[p] = -;
   return lim - rest;
 }
 #undef y

 int Dinic() {
   int res = ;
   while (bfs())
     res += dfs(S, inf_int);
   return res;
 }

 inline bool check(ll t) {
     static int i, j;
     memset(first, , sizeof(first)), tot = ;
     for (i = ; i <= n; ++i)
         Add_Edges(S, i *  - , a[i]), Add_Edges(i * , T, b[i]);
     for (i =  ; i <= n; ++i)
         for (j = ; j <= n; ++j)
             if (mp[i][j] <= t) Add_Edges(i *  - , j * , inf_int);
     return Dinic() == sum;
 }

 int main() {
     int i, j, k, x, y, z;
     ll l, r, mid;
     n = read(), m = read(), S = n *  + , T = S + ;
     for (i = ; i <= n; ++i)
         sum += (a[i] = read()), b[i] = read();
     for (i = ; i <= n; ++i)
         for (j = ; j <= n; ++j) mp[i][j] = i == j ?  : inf_ll;
     for (i = ; i <= m; ++i) {
         x = read(), y = read(), z = read();
         if (mp[x][y] > z) mp[x][y] = mp[y][x] = z;
     }
     for (k = ; k <= n; ++k)
         for (i = ; i <= n; ++i)
             for (j = ; j <= n; ++j)
                 mp[i][j] = min(mp[i][j], mp[i][k] + mp[k][j]);

     l = , r = inf_ll;
     while (l +  < r) {
         mid = l + r >> ;
         if (check(mid)) r = mid;
         else l = mid;
     }
     printf("%lld\n", r == inf_ll ? -1ll : r);
     return ;
 }

 inline int read() {
     static int x;
     static char ch;
     x = , ch = getchar();
     while (ch < '' || '' < ch)
         ch = getchar();
     while ('' <= ch && ch <= '') {
         x = x *  + ch - '';
         ch = getchar();
     }
     return x;
 }