NWERC 2012 A - Admiral【最小费用最大流】

  1 //已ac
  2 #include <iostream>
  3 #include <cstdio>
  4 #include <cstring>
  5 #include <vector>
  6 #include <queue>
  7 using namespace std;
  8 
  9 const int maxn = 10005;
 10 const int INF = 1000000000;
 11 
 12 struct Edge {
 13     int from, to, cap, flow, cost;//起点 终点 容量 花费
 14 };
 15 
 16 struct MCMF
 17 {
 18     int n, m, s, t;
 19     vector<Edge> edges;
 20     vector<int> G[maxn];
 21 
 22     int inq[maxn];
 23     int d[maxn];
 24     int p[maxn];
 25     int a[maxn];
 26 
 27     void init(int n) {//初始化的是所有的点数
 28         this -> n = n;
 29         for(int i = 0; i < n; i++) G[i].clear();
 30         edges.clear();
 31     }
 32 
 33     void AddEdge(int from, int to, int cap, int cost) {
 34         edges.push_back((Edge) { from, to, cap, 0, cost } );
 35         edges.push_back((Edge) { to, from, 0, 0, -cost } );
 36         m = edges.size();
 37         G[from].push_back(m - 2);
 38         G[to].push_back(m - 1);
 39     }
 40 
 41     bool BellmanFord(int s, int t, int &flow, int &cost) {
 42         for(int i = 0; i < n; i++) d[i] = INF;
 43         memset(inq, 0, sizeof(inq) );
 44         d[s] = 0; inq[s] = 1; p[s] = 0; a[s] = INF;
 45         queue<int> Q;
 46         Q.push(s);
 47         while(!Q.empty()) {
 48             int u = Q.front(); Q.pop();
 49             inq[u] = 0;
 50             for(int i = 0; i < G[u].size(); i++) {
 51                 Edge &e = edges[G[u][i]];
 52                 if(e.cap > e.flow && d[e.to] > d[u] + e.cost) {
 53                     d[e.to] = d[u] + e.cost;
 54                     p[e.to] = G[u][i];
 55                     a[e.to] = min(a[u], e.cap - e.flow);
 56                     if(!inq[e.to]) { Q.push(e.to); inq[e.to] = 1; }
 57                 }
 58             }
 59         }
 60 
 61         if(d[t] == INF) return false;
 62         flow += a[t];
 63         cost += d[t] * a[t];
 64         int u = t;
 65         while(u != s) {
 66             edges[p[u]].flow += a[t];
 67             edges[p[u] ^ 1].flow -= a[t];
 68             u = edges[p[u]].from;
 69         }
 70         return true;
 71     }
 72 
 73     int MinCost(int s, int t) {//起点和终点
 74       //  this -> s = s; this -> t = t;
 75         int flow = 0, cost = 0;
 76         while(BellmanFord(s, t, flow, cost)){};
 77         return cost;
 78     }
 79 };
 80 
 81 MCMF g;
 82 
 83 int main() {
 84     int n, m;
 85     int a, b, c;
 86     freopen("a.txt","r",stdin);
 87     while(EOF != scanf("%d %d",&n, &m)) {
 88         g.init(n * 2 + 1);
 89         g.AddEdge(1, 1 + n, 2, 0);
 90         g.AddEdge(n, n + n, 2, 0);
 91         for(int i = 2; i <= n - 1; i++) {
 92             g.AddEdge(i, i + n, 1, 0);
 93         }
 94         for(int i = 0; i < m; i ++) {
 95             scanf("%d %d %d",&a, &b, &c);
 96             g.AddEdge(a + n, b, 1, c);
 97         }
 98         printf("%d\n",g.MinCost(1, n + n));
 99     }
100     return 0;
101 }