HDU 4280 Island Transport(网络流)

有N个岛屿 M条无向路每个路有一最大允许的客流量，求从最西的那个岛屿最多能运用多少乘客到最东的那个岛屿。
  1 #include <iostream>
  2 #include <cstring>
  3 #include <cstdio>
  4 #include <queue>
  5 #define M 410000//边
  6 #define N 110000//点
  7 using namespace std;
  8 #define INF 0x3f3f3f3f
  9 struct edge
 10 {
 11     int from;
 12     int to;
 13     int c;
 14     int next;
 15 }edge[M];
 16 int ant,head[N],dep[N],gap[N];
 17 void add(int a,int b,int c)
 18 {
 19     edge[ant].from=a;
 20     edge[ant].to=b;
 21     edge[ant].c=c;
 22     edge[ant].next=head[a];
 23     head[a]=ant++;
 24     edge[ant].from=b;
 25     edge[ant].to=a;
 26     edge[ant].c=0;
 27     edge[ant].next=head[b];
 28     head[b]=ant++;
 29 }
 30 void BFS(int start,int end)
 31 {
 32     int i,to,u;
 33     memset(dep,-1,sizeof(dep));
 34     memset(gap,0,sizeof(gap));
 35     queue < int > que ;
 36     dep[end]=0;
 37     gap[0]=1;
 38     que.push(end);
 39     while(!que.empty())
 40     {
 41         u=que.front();
 42         que.pop();
 43         for(i=head[u];i!=-1;i=edge[i].next)
 44         {
 45             to=edge[i].to;
 46             if(dep[to]!=-1||edge[i].c!=0)
 47                 continue;
 48             que.push(to);
 49             dep[to]=dep[u]+1;
 50             gap[dep[to]]++;
 51         }
 52     }
 53 }
 54 int s[N],cur[N],n;
 55 int sap(int start,int end)
 56 {
 57     int res=0;
 58     BFS(start,end);
 59     int u=start,top=0;
 60     memcpy(cur,head,sizeof(head));
 61     while(dep[start]<n)
 62     {
 63         int i;
 64         if(u==end)
 65         {
 66             int Min=INF,flag;
 67             for(i=0;i<top;i++)
 68             {
 69                 if(edge[s[i]].c<Min)
 70                 {
 71                     Min=edge[s[i]].c;
 72                     flag=i;
 73                 }
 74             }
 75             res+=Min;
 76             for(i=0;i<top;i++)
 77             {
 78                 edge[s[i]].c-=Min;
 79                 edge[s[i]^1].c+=Min;
 80             }
 81             top=flag;
 82             u=edge[s[top]].from;
 83         }
 84         if(u!=end&&gap[dep[u]-1]==0)//无增广路
 85             break;
 86         for(i=cur[u];i!=-1;i=edge[i].next)
 87         {
 88             if(edge[i].c!=0&&dep[edge[i].to]+1==dep[u])
 89                 break;
 90         }
 91         if(i!=-1)
 92         {
 93             cur[u]=i;
 94             s[top++]=i;
 95             u=edge[i].to;
 96         }
 97         else
 98         {
 99             int Min=n;
100             for(i=head[u];i!=-1;i=edge[i].next)
101             {
102                 if(edge[i].c==0)
103                     continue;
104                 if(Min>dep[edge[i].to])
105                 {
106                     cur[u]=i;
107                     Min=dep[edge[i].to];
108                 }
109             }
110             gap[dep[u]]--;
111             dep[u]=Min+1;
112             gap[dep[u]]++;
113             if(u!=start)
114                 u=edge[s[--top]].from;
115         }
116     }
117     return res;
118 }
119 int main()
120 {
121     int m,i;
122     int minx,maxx,x,y,start,end,t,c;
123     scanf("%d",&t);
124     while(t--)
125     {
126         minx=INF,maxx=-INF;
127         scanf("%d%d",&n,&m);
128         for(i=1;i<=n;i++)
129         {
130             scanf("%d%d",&x,&y);
131             if(x<=minx)
132             start=i,minx=x;
133             if(x>=maxx)
134             end=i,maxx=x;
135         }
136         ant=0;
137         memset(head,-1,sizeof(head));
138         for(i=1;i<=m;i++)
139         {
140             scanf("%d%d%d",&x,&y,&c);
141             add(x,y,c);
142             add(y,x,c);
143         }
144         printf("%d\n",sap(start,end));
145     }
146     return 0;
147 }
posted on 2012-09-30 16:22 pony1993 阅读(159) 评论(0) 编辑收藏举报