【洛谷 p3376】模板-网络最大流（图论）

 1 #include<cstdio>
 2 #include<cstdlib>
 3 #include<cstring>
 4 #include<iostream>
 5 #include<queue>
 6 using namespace std;
 7 
 8 const int N=10010,M=100010,INF=(int)1e9;
 9 int n,m,st,ed,len=1;
10 int last[N],d[N];
11 struct edge{int y,fl,next;}a[2*M];
12 queue<int> q;
13 
14 int mmin(int x,int y) {return x<y?x:y;}
15 void ins(int x,int y,int fl)
16 {
17     a[++len].y=y,a[len].fl=fl;
18     a[len].next=last[x],last[x]=len;
19     a[++len].y=x,a[len].fl=0;
20     a[len].next=last[y],last[y]=len;
21 }
22 bool bfs()
23 {
24     while (!q.empty()) q.pop();
25     memset(d,-1,sizeof(d));
26     q.push(st); d[st]=1;
27     while (!q.empty())
28     {
29       int x=q.front(); q.pop();
30       for (int i=last[x];i!=-1;i=a[i].next)
31       {
32         int y=a[i].y;
33         if (!a[i].fl || d[y]!=-1) continue;
34         d[y]=d[x]+1; q.push(y);
35       }
36     }
37     return (d[ed]!=-1);
38 }
39 int dfs(int x,int flow)
40 {
41     if (x==ed) return flow;
42     int sum=0;
43     for (int i=last[x];i!=-1;i=a[i].next)
44     {
45       int y=a[i].y;
46       if (d[y]!=d[x]+1 || !a[i].fl) continue;
47       int p=mmin(a[i].fl,flow-sum);
48       int tmp=dfs(y,p);
49       sum+=tmp;
50       a[i].fl-=tmp,a[i^1].fl+=tmp;
51       if (sum==flow) break;
52     }
53     if (!sum) d[x]=-1;
54     return sum;
55 }
56 int Max_flow()
57 {
58     int sum=0,p;
59     while (bfs())
60       while (p=dfs(st,INF)) sum+=p;
61     return sum;
62 }
63 int main()
64 {
65     scanf("%d%d%d%d",&n,&m,&st,&ed);
66     int x,y,d;
67     memset(last,-1,sizeof(last));
68     for (int i=1;i<=m;i++)
69     {
70       scanf("%d%d%d",&x,&y,&d);
71       ins(x,y,d);
72     }
73     printf("%d
",Max_flow());
74     return 0;
75 }