POJ 1273 Drainage Ditches(最大流模版EK+dinic）

题目链接

熟悉这两个模版。这两个思路差不多，实现方式有所不同，好像dinic应该快一点，但是在POJ上都是16ms....

dinic算法

 1 #include <cstdio>//dinic算法

 2 #include <cstring>

 3 #include <cmath>

 4 #include <queue>

 5 using namespace std;

 6 #define INF 0x7fffffff

 7 queue<int> que;

 8 int flow[201][201],dis[201];

 9 int n,m;

10 int bfs()

11 {

12     int t,i;

13     memset(dis,-1,sizeof(dis));

14     while(!que.empty()) que.pop();

15     que.push(1);

16     dis[1] = 0;

17     while(!que.empty())

18     {

19         t = que.front();

20         que.pop();

21         for(i = 1;i <= n;i ++)

22         {

23             if(flow[t][i] > 0&&dis[i] < 0)

24             {

25                 dis[i] = dis[t]+1;

26                 que.push(i);

27             }

28         }

29     }

30     if(dis[n] > 0) return 1;

31     else  return 0;

32 }

33 int dfs(int x,int step)

34 {

35     int i,t;

36     if(x == n) return step;

37     for(i = 1;i <= n;i ++)

38     {

39         if(flow[x][i] > 0&&dis[i] == dis[x]+1&&(t = dfs(i,min(step,flow[x][i]))))

40         {

41             flow[x][i] -= t;

42             flow[i][x] += t;

43             return t;

44         }

45     }

46     return 0;

47 }

48 int main()

49 {

50     int i,sv,ev,w,res,ans;

51     while(scanf("%d%d",&m,&n)!=EOF)

52     {

53         ans = 0;

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

55         for(i = 1;i <= m;i ++)

56         {

57             scanf("%d%d%d",&sv,&ev,&w);

58             flow[sv][ev] += w;

59         }

60         while(bfs())

61         {

62            while(res = dfs(1,INF))

63            ans += res;

64         }

65         printf("%d\n",ans);

66     }

67     return 0;

68 }

 

EK算法

 1 #include <cstdio>

 2 #include <cstring>

 3 #include <cmath>

 4 #include <queue>

 5 using namespace std;

 6 #define INF 0x7fffffff

 7 queue<int> que;

 8 int flow[201][201],low[201],path[201];

 9 int str,end,n,m;

10 int bfs()

11 {

12     int t,i;

13     memset(path,-1,sizeof(path));

14     while(!que.empty()) que.pop();

15     que.push(str);

16     low[str] = INF;

17     while(!que.empty())

18     {

19         t = que.front();

20         que.pop();

21         if(t == end) break;

22         for(i = 1;i <= n;i ++)

23         {

24             if(i != str&&path[i]==-1&&flow[t][i])

25             {

26                 low[i] = low[t] < flow[t][i] ? low[t]:flow[t][i];

27                 path[i] = t;

28                 que.push(i);

29             }

30         }

31     }

32     if(path[end] == -1)

33     return -1;

34     else

35     return low[end];

36 }

37 int EK()

38 {

39     int ans = 0,res,now;

40     while((res = bfs()) != -1)

41     {

42         ans += res;

43         now = end;

44         while(now != str)

45         {

46             flow[now][path[now]] += res;

47             flow[path[now]][now] -= res;

48             now = path[now];

49         }

50     }

51     return ans;

52 }

53 int main()

54 {

55     int i,sv,ev,w;

56     while(scanf("%d%d",&m,&n)!=EOF)

57     {

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

59         for(i = 1;i <= m;i ++)

60         {

61             scanf("%d%d%d",&sv,&ev,&w);

62             flow[sv][ev] += w;

63         }

64         str = 1,end = n;

65         printf("%d\n",EK());

66     }

67     return 0;

68 }