题目：给出一个网络图，以及其源点和汇点，求出其网络最大流。
解法：网络流Dinic算法。
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 }