POJ 1273 (基础最大流) Drainage Ditches

 1 #include <cstdio>
 2 #include <cstring>
 3 #include <algorithm>
 4 #include <vector>
 5 #include <queue>
 6 using namespace std;
 7 
 8 const int maxn = 200 + 10;
 9 const int INF = 1000000000;
10 
11 struct Edge
12 {
13     int from, to, cap, flow;
14     Edge(int u, int v, int c, int f):from(u), to(v), cap(c), flow(f) {}
15 };
16 
17 int n, m, M;
18 vector<Edge> edges;
19 vector<int> G[maxn];
20 int a[maxn];    //到起点i的可改进量
21 int p[maxn];    //最短路数上p的入弧编号
22 
23 void Init(int n)
24 {
25     for(int i = 0; i < n; ++i) G[i].clear();
26     edges.clear();
27 }
28 
29 void AddEdge(int from, int to, int cap)
30 {
31     edges.push_back(Edge(from, to, cap, 0));
32     edges.push_back(Edge(to, from, 0, 0));    //反向弧
33     m = edges.size();
34     G[from].push_back(m-2);
35     G[to].push_back(m-1);
36 }
37 
38 int MaxFlow(int s, int t)
39 {
40     int flow = 0;
41     for(;;)
42     {
43         memset(a, 0, sizeof(a));
44         queue<int> Q;
45         Q.push(s);
46         a[s] = INF;
47         while(!Q.empty())
48         {
49             int x = Q.front(); Q.pop();
50             for(int i = 0; i < G[x].size(); ++i)
51             {
52                 Edge& e = edges[G[x][i]];
53                 if(!a[e.to] && e.cap > e.flow)
54                 {
55                     p[e.to] = G[x][i];
56                     a[e.to] = min(a[x], e.cap-e.flow);
57                     Q.push(e.to);
58                 }
59             }
60             if(a[t]) break;
61         }
62         if(!a[t]) break;
63         for(int u = t; u != s; u = edges[p[u]].from)
64         {
65             edges[p[u]].flow += a[t];
66             edges[p[u]^1].flow -= a[t];
67         }
68         flow += a[t];
69     }
70     return flow;
71 }
72 
73 int main()
74 {
75     freopen("in.txt", "r", stdin);
76 
77     while(scanf("%d%d", &M, &n) == 2)
78     {
79         Init(n);
80         int u, v, c;
81         for(int i = 0; i < M; ++i)
82         {
83             scanf("%d%d%d", &u, &v, &c);
84             AddEdge(u-1, v-1, c);
85         }
86         printf("%d\n", MaxFlow(0, n-1));
87     }
88 
89     return 0;
90 }