网络流/poj1273 Drainage Ditches
题意
一个有向图，给出n个点，m条边，给出每条边的起点和中点以及容量，求最大流
分析
求最大流模板
Accepted Code
dinic
1 { 2 PROBLEM:poj 1273 3 AUTHER:Rinyo 4 MEMO:网络流 5 } 6 Program flow; 7 Const 8 Infile = 'flow.out'; 9 Outfile = 'flow.out'; 10 Var 11 q:Array[0..100000]Of Longint; 12 map:Array[0..400,0..400]Of Longint; 13 num:Array[0..400]Of Longint; 14 sum,i,n,m,x,y,w:Longint; 15 flag:Boolean; 16 17 Function min(a,b:Longint):Longint; 18 Begin 19 If a<b Then min:=a Else min:=b; 20 End; 21 22 Function dfs(now,value:Longint):Longint; 23 Var 24 v1,x,i:Longint; 25 Begin 26 If (now=n) Then Begin 27 dfs:=value; 28 Exit; 29 End; 30 v1:=value; 31 For i:=1 To n Do 32 If (map[now,i]>0) And (num[now]+1=num[i]) Then Begin 33 x:=dfs(i,min(value,map[now,i])); 34 value:=value-x; 35 Dec(map[now,i],x); 36 Inc(map[i,now],x); 37 End; 38 dfs:=v1-value; 39 End; 40 41 Function bfs:Boolean; 42 Var 43 head,tail,now,i:Longint; 44 Begin 45 Fillchar(q,sizeof(q),0); 46 q[1]:=1;head:=0;tail:=1;flag:=false; 47 Fillchar(num,sizeof(num),0); 48 num[1]:=1; 49 While head<tail Do Begin 50 Inc(head); 51 now:=q[head]; 52 For i:=1 To n Do 53 If map[now,i]>0 Then 54 If num[i]=0 Then Begin 55 Inc(tail); 56 q[tail]:=i; 57 num[i]:=num[now]+1; 58 If i=n Then begin 59 flag:=true; 60 Exit; 61 End; 62 End; 63 End; 64 End; 65 66 Begin 67 Assign(input,infile);Reset(input); 68 Assign(output,outfile);Rewrite(output); 69 While not eof Do Begin 70 ReadLn(m,n); 71 Fillchar(map,sizeof(map),0); 72 For i:=1 To m Do begin 73 ReadLn(x,y,w); 74 map[x,y]:=map[x,y]+w; 75 End; 76 flag:=true; 77 sum:=0; 78 While flag Do Begin 79 bfs; 80 sum:=sum+dfs(1,maxlongint); 81 End; 82 WriteLn(sum); 83 End; 84 Close(input);Close(output); 85 End.
sap
1 { 2 PROBLEM:poj 1273 3 AUTHER:Rinyo 4 MEMO:网络流 5 } 6 7 Program flow; 8 Const 9 Infile = 'flow.in'; 10 Outfile = 'flow.out'; 11 Var 12 maxflow,n,m,i,x,y,z:Longint; 13 map,temp:Array[0..430,0..430]Of Longint; 14 num,q,pre:Array[0..10000]Of Longint; 15 16 Function min(a,b:Longint):Longint; 17 Begin 18 if a<b Then Exit(a) Else Exit(b); 19 End; 20 21 Procedure bfs; 22 Var 23 head,tail,i,now:Longint; 24 Begin 25 Fillchar(q,sizeof(q),0); 26 q[1]:=n;head:=0;tail:=1;num[n]:=0; 27 Fillchar(num,sizeof(num),0); 28 While head<tail Do Begin 29 Inc(head); 30 now:=q[head]; 31 For i:=1 To n Do 32 if (temp[now,i]>0) And (num[i]=0) Then Begin 33 Inc(tail); 34 q[tail]:=i; 35 num[i]:=num[now]+1; 36 if i=n Then Exit; 37 End; 38 End; 39 End; 40 41 Function findallowed(i:Longint):Longint; 42 Var 43 j:Longint; 44 Begin 45 For j:=1 To n Do 46 If (map[i,j]>0) And (num[i]=num[j]+1) Then Exit(j); 47 Exit(-1); 48 End; 49 50 Function relable(i:Longint):Longint; 51 Var 52 t,j:Longint; 53 Begin 54 t:=maxlongint; 55 For j:=1 To n Do 56 if map[i,j]>0 Then t:=min(t,num[j]+1); 57 If t=maxlongint Then Exit(n) Else Exit(t); 58 End; 59 60 Function sap:Longint; 61 Var 62 i,j,ll,x:Longint; 63 Begin 64 maxflow:=0;i:=1; 65 While (num[1]<n) Do Begin 66 j:=findallowed(i); 67 If (j>=0) Then Begin 68 pre[j]:=i;i:=j; 69 If (i=n) Then Begin 70 ll:=maxlongint; 71 i:=n; 72 While i<>1 Do BEgin 73 ll:=min(ll,map[pre[i],i]); 74 i:=pre[i]; 75 End; 76 i:=n; 77 While i<>1 Do Begin 78 map[pre[i],i]:=map[pre[i],i]-ll; 79 map[i,pre[i]]:=map[i,pre[i]]+ll; 80 i:=pre[i]; 81 End; 82 maxflow:=maxflow+ll; 83 End; 84 End Else Begin 85 x:=relable(i); 86 num[i]:=x; 87 While i<>1 Do i:=pre[i]; 88 End; 89 End; 90 End; 91 92 Begin 93 Assign(input,infile);Reset(input); 94 Assign(output,outfile);Rewrite(output); 95 While not eof Do begin 96 Fillchar(map,sizeof(map),0); 97 Fillchar(temp,sizeof(temp),0); 98 Fillchar(pre,sizeof(pre),$ff); 99 Fillchar(num,sizeof(num),0); 100 ReadLn(m,n); 101 For i:=1 To m Do Begin 102 ReadLn(x,y,z); 103 map[x,y]:=map[x,y]+z; 104 temp[y,x]:=temp[y,x]+z; 105 End; 106 bfs; 107 sap; 108 WriteLn(maxflow); 109 End; 110 Close(input);Close(output); 111 End.
