poj2455Secret Milking Machine【二分 + 最大流】

  1 #include <iostream>
  2 #include <queue>
  3 #include <cstdio>
  4 #include <cstring>
  5 #include <vector>
  6 using namespace std;
  7 
  8 const int maxn = 40005;
  9 const int INF = 1000000000;
 10 
 11 struct Edge {
 12     int from, to, cap, flow;
 13 };
 14 
 15 struct Dinic
 16 {
 17     int n, m, s, t;
 18     vector<Edge> edges;
 19     vector<int> G[maxn];
 20     bool vis[maxn];
 21     int d[maxn];
 22     int cur[maxn];
 23 
 24     void ClearAll(int n) {
 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         edges.push_back((Edge) { from, to, cap, 0});
 31         edges.push_back((Edge) { to, from, 0, 0} );
 32         m = edges.size();
 33         G[from].push_back(m - 2);
 34         G[to].push_back(m - 1);
 35     }
 36 
 37     bool BFS() {
 38         memset(vis, 0, sizeof(vis));
 39         queue<int> Q;
 40         Q.push(s);
 41         d[s] = 0;
 42         vis[s] = 1;
 43         while(!Q.empty()) {
 44             int x = Q.front(); Q.pop();
 45             for(int i = 0; i < G[x].size(); i++) {
 46                 Edge& e = edges[G[x][i]];
 47                 if(!vis[e.to] && e.cap > e.flow) {
 48                     vis[e.to] = 1;
 49                     d[e.to] = d[x] + 1;
 50                     Q.push(e.to);
 51                 }
 52             }
 53         }
 54         return vis[t];
 55     }
 56 
 57     int DFS(int x, int a) {
 58         if(x == t || a == 0) return a;
 59         int flow = 0, f;
 60         for(int& i = cur[x]; i < G[x].size(); i++) {
 61             Edge& e = edges[G[x][i]];
 62             if(d[x] + 1 == d[e.to] && (f = DFS(e.to, min(a, e.cap - e.flow))) > 0) {
 63                 e.flow += f;
 64                 edges[G[x][i]^1].flow -= f;
 65                 flow += f;
 66                 a -= f;
 67                 if(a == 0) break;
 68             }
 69         }
 70         return flow;
 71     }
 72 
 73     int MaxFlow(int s, int t) {
 74         this -> s = s; this -> t = t;
 75         int flow = 0;
 76         while(BFS()) {
 77             memset(cur, 0, sizeof(cur));
 78             flow += DFS(s, INF);
 79         }
 80         return flow;
 81     }
 82 };
 83 
 84 Dinic g;
 85 
 86 int N, P, T, s, t;
 87 
 88 struct Point {
 89     int x, y, z;
 90 }point[maxn];
 91 
 92 bool check(int mid) {
 93     //printf("*%d\n",mid);
 94     g.ClearAll(maxn);
 95     g.AddEdge(s, 1, T);
 96     g.AddEdge(N, t, T);
 97     for(int i = 0; i < P; i++) {
 98         if(point[i].z <= mid) {
 99             g.AddEdge(point[i].x, point[i].y, 1);
100             g.AddEdge(point[i].y, point[i].x, 1);
101             //printf("#%d %d %d\n",point[i].x, point[i].y, point[i].z);
102         }
103     }
104     int ans = g.MaxFlow(s, t);
105     //printf("%d\n",ans);
106     if(ans != T) return true;
107     else return false;
108 }
109 
110 int main() {
111     //freopen("a.txt","r",stdin);
112     while(EOF != scanf("%d %d %d",&N, &P, &T)) {
113         s = 0; t = N + 1;
114         //printf("%d %d\n",s, t);
115         int max_num = -INF, min_num = INF;
116         for(int i = 0; i < P; i++) {
117             scanf("%d %d %d",&point[i].x, &point[i].y, &point[i].z);
118             if(max_num < point[i].z) max_num = point[i].z;
119             if(min_num > point[i].z) min_num = point[i].z;
120         }
121 
122         int low = min_num; int high = max_num;
123         while(low <= high) {
124             int mid = (low + high) >> 1;
125             //printf("*%d\n",mid);
126             if(check(mid))
127                 low = mid + 1;
128             else
129                 high = mid - 1;
130         }
131         printf("%d\n",high + 1);
132     }
133     return 0;
134 }