UVA 820 Internet Bandwidth 因特网宽带（无向图，最大流，常规）

 

 

题意：给一个无向图，每条边上都有容量的限制，要求求出给定起点和终点的最大流。

思路：每条无向边就得拆成2条，每条还得有反向边，所以共4条。源点汇点已经给出，所以不用建了。直接在图上跑最大流就可以了。

 

 

 1 #include <bits/stdc++.h>

 2 #define LL long long

 3 #define pii pair<int,int>

 4 #define INF 0x7f7f7f7f

 5 using namespace std;

 6 const int N=200;

 7 const int mod=1e9+7;

 8 int s, t;

 9 

10 int path[N], flow[N];

11 vector<int> vect[N];

12 

13 struct node

14 {

15     int from, to, cap, flow;

16     node(){};

17     node(int from,int to,int cap,int flow):from(from),to(to),cap(cap),flow(flow){};

18 }edge[100000];

19 int edge_cnt;

20 

21 void add_node(int from,int to,int cap,int flow)

22 {

23     edge[edge_cnt]=node(from,to,cap,flow);

24     vect[from].push_back(edge_cnt++);

25 }

26 

27 int BFS(int s,int e)

28 {

29     deque<int> que(1,s);

30     flow[s]=INF;

31     while(!que.empty())

32     {

33         int x=que.front();

34         que.pop_front();

35         for(int i=0; i<vect[x].size(); i++)

36         {

37             node e=edge[vect[x][i]];

38             if(!flow[e.to] && e.cap>e.flow)

39             {

40                 flow[e.to]=min(flow[e.from],e.cap-e.flow);

41                 path[e.to]=vect[x][i];

42                 que.push_back(e.to);

43             }

44         }

45         if(flow[e]) return flow[e];

46     }

47     return flow[e];

48 }

49 

50 int max_flow(int s,int e)

51 {

52     int ans_flow=0;

53     while(true)

54     {

55         memset(path,0,sizeof(path));

56         memset(flow,0,sizeof(flow));

57 

58         int tmp=BFS(s,e);

59         if(!tmp)    return ans_flow;

60         ans_flow+=tmp;

61 

62         int ed=e;

63         while(ed!=s)

64         {

65             int t=path[ed];

66             edge[t].flow+=tmp;

67             edge[t^1].flow-=tmp;

68             ed=edge[t].from;

69         }

70     }

71 }

72 int main()

73 {

74     freopen("input.txt", "r", stdin);

75     int n, a, b, v, c, j=0;

76     while(scanf("%d",&n),n)

77     {

78         edge_cnt=0;

79         memset(edge,0,sizeof(edge));

80         for(int i=0; i<=n+1; i++)   vect[i].clear();

81 

82         scanf("%d%d%d", &s, &t, &c);

83         for(int i=0; i<c; i++)

84         {

85             scanf("%d%d%d",&a,&b,&v);

86             add_node(a, b, v, 0);

87             add_node(b, a, 0, 0);

88             add_node(b, a, v, 0);

89             add_node(a, b, 0, 0);

90         }

91         printf("Network %d\n",++j);

92         printf("The bandwidth is %d.\n\n", max_flow(s ,t) );

93     }

94     return 0;

95 }

AC代码