P1345 [USACO5.4]奶牛的电信Telecowmunication【最小割】【最大流】

  1 #include <bits/stdc++.h>
  2 #define dbg(x) cout << #x << "=" << x << endl
  3 #define eps 1e-8
  4 #define pi acos(-1.0)
  5 
  6 using namespace std;
  7 typedef long long LL;
  8 
  9 template<class T>inline void read(T &res)
 10 {
 11     char c;T flag=1;
 12     while((c=getchar())<‘0‘||c>‘9‘)if(c==‘-‘)flag=-1;res=c-‘0‘;
 13     while((c=getchar())>=‘0‘&&c<=‘9‘)res=res*10+c-‘0‘;res*=flag;
 14 }
 15 
 16 namespace _buff {
 17     const size_t BUFF = 1 << 19;
 18     char ibuf[BUFF], *ib = ibuf, *ie = ibuf;
 19     char getc() {
 20         if (ib == ie) {
 21             ib = ibuf;
 22             ie = ibuf + fread(ibuf, 1, BUFF, stdin);
 23         }
 24         return ib == ie ? -1 : *ib++;
 25     }
 26 }
 27 
 28 int qread() {
 29     using namespace _buff;
 30     int ret = 0;
 31     bool pos = true;
 32     char c = getc();
 33     for (; (c < ‘0‘ || c > ‘9‘) && c != ‘-‘; c = getc()) {
 34         assert(~c);
 35     }
 36     if (c == ‘-‘) {
 37         pos = false;
 38         c = getc();
 39     }
 40     for (; c >= ‘0‘ && c <= ‘9‘; c = getc()) {
 41         ret = (ret << 3) + (ret << 1) + (c ^ 48);
 42     }
 43     return pos ? ret : -ret;
 44 }
 45 
 46 const int maxn = 1e4 + 7;
 47 const int inf = 0x3f3f3f3f;
 48 
 49 int n,m,cnt = 1,s,t;
 50 
 51 int maxflow;
 52 int head[maxn];
 53 
 54 struct Node {
 55     int v,dis,nxt;
 56 }edge[maxn << 2];
 57 
 58 void BuildGraph(int u,int v,int dis){
 59     edge[++cnt].nxt = head[u];
 60     edge[cnt].v = v;
 61     edge[cnt].dis = dis;
 62     head[u] = cnt;
 63 
 64     edge[++cnt].nxt = head[v];
 65     edge[cnt].v = u;
 66     edge[cnt].dis = 0;
 67     head[v] = cnt;
 68 }
 69 
 70 int depth[maxn];
 71 
 72 bool bfs() {
 73     queue<int> q;
 74     while(!q.empty())
 75         q.pop();
 76     memset(depth,0,sizeof(depth));
 77     depth[s] = 1;
 78     q.push(s);
 79     while(!q.empty()){
 80         int u = q.front();
 81         q.pop();
 82         for(int i = head[u]; i; i = edge[i].nxt){
 83             int v = edge[i].v;
 84             if(edge[i].dis && !depth[v]){
 85                 depth[v] = depth[u] + 1;
 86                 if(v == t)
 87                     return true;
 88                 q.push(v);
 89             }
 90         }
 91     }
 92     return false;
 93 }
 94 
 95 int Dinic(int u,int flow) {//最大流
 96     if(u == t)
 97         return flow;
 98     int rest = flow,temp;//残量网络
 99     for(int i = head[u];i;i = edge[i].nxt) {
100         int v = edge[i].v;
101         if(depth[v] == depth[u] + 1 && edge[i].dis){
102             temp = Dinic(v,min(rest,edge[i].dis));
103             if(!temp)depth[v] = 0;
104             rest -= temp;
105             edge[i].dis -= temp;
106             edge[i ^ 1].dis += temp;
107         }
108     }
109     return flow - rest;
110 }
111 
112 int main(){
113     scanf("%d %d %d %d",&n, &m, &s, &t);
114     for(int i = 1;i <= n;i++){
115         BuildGraph(i, i + n, 1);
116     }
117 
118     int u,v;
119     for(int i = 1; i <= m; i++){
120         scanf("%d %d",&u, &v);
121         BuildGraph(u + n,v,inf);
122         BuildGraph(v + n,u,inf);
123     }
124     s += n;
125     int flow = 0;
126     while(bfs()){
127         while(flow = Dinic(s,inf))
128             maxflow += flow;
129     }
130     printf("%d\n",maxflow);
131     return 0;
132 }