从今天开始学习网络流.jpg
用的Dinic
复杂度O(能过)
1 //minamoto 2 #include<iostream> 3 #include<cstdio> 4 #include<algorithm> 5 #include<cstring> 6 #include<queue> 7 using namespace std; 8 #define getc() (p1==p2&&(p2=(p1=buf)+fread(buf,1,1<<21,stdin),p1==p2)?EOF:*p1++) 9 char buf[1<<21],*p1=buf,*p2=buf; 10 inline int read(){ 11 #define num ch-'0' 12 char ch;bool flag=0;int res; 13 while(!isdigit(ch=getc())) 14 (ch=='-')&&(flag=true); 15 for(res=num;isdigit(ch=getc());res=res*10+num); 16 (flag)&&(res=-res); 17 #undef num 18 return res; 19 } 20 char sr[1<<21],z[20];int C=-1,Z; 21 inline void Ot(){fwrite(sr,1,C+1,stdout),C=-1;} 22 inline void print(int x){ 23 if(C>1<<20)Ot();if(x<0)sr[++C]=45,x=-x; 24 while(z[++Z]=x%10+48,x/=10); 25 while(sr[++C]=z[Z],--Z);sr[++C]=' '; 26 } 27 const int inf=0x3f3f3f3f,N=10005,M=200005; 28 int n,m,u,v,e,mxflow,dep[N]; 29 int head[N],Next[M],ver[M],edge[M]; 30 int tot=1,cur[N],s,t; 31 queue<int> q; 32 inline void add(int u,int v,int e){ 33 ver[++tot]=v,Next[tot]=head[u],head[u]=tot,edge[tot]=e; 34 ver[++tot]=u,Next[tot]=head[v],head[v]=tot,edge[tot]=0; 35 } 36 bool bfs(){ 37 memset(dep,-1,sizeof(dep)); 38 while(!q.empty()) q.pop(); 39 for(int i=1;i<=n;++i) cur[i]=head[i]; 40 dep[s]=0,q.push(s); 41 while(!q.empty()){ 42 int u=q.front();q.pop(); 43 for(int i=head[u];i;i=Next[i]){ 44 int v=ver[i]; 45 if(dep[v]<0&&edge[i]) 46 dep[v]=dep[u]+1,q.push(v); 47 } 48 } 49 if(~dep[t]) return true; 50 return false; 51 } 52 int dfs(int u,int limit){ 53 if(!limit||u==t) return limit; 54 int flow=0,f; 55 //这里遍历的时候要写cur而不是head 56 //为了玄学的弧优化 57 for(int i=cur[u];i;i=Next[i]){ 58 cur[u]=i;int v=ver[i]; 59 if(dep[v]==dep[u]+1&&(f=dfs(v,min(limit,edge[i])))){ 60 flow+=f,limit-=f; 61 edge[i]-=f,edge[i^1]+=f; 62 if(!limit) break; 63 } 64 } 65 return flow; 66 } 67 void dinic(){ 68 while(bfs()) mxflow+=dfs(s,inf); 69 } 70 int main(){ 71 n=read(),m=read(),s=read(),t=read(); 72 for(int i=1;i<=m;++i){ 73 int u=read(),v=read(),e=read(); 74 add(u,v,e); 75 } 76 dinic(); 77 print(mxflow); 78 Ot(); 79 return 0; 80 }