nyoj Arbitrage (Bellman-Ford)

跟天下第一挺像的

 1 void Bellman(int v0)

 2 {

 3     int i, k;

 4     for (int i = 0; i < n; ++i) {

 5         dist[i] = INF, path[i] = -1;

 6     }

 7     dist[v0] = 0;

 8     for (k = 1; k < n; ++k) {

 9         for (i = 0; i < m; ++i) {

10             if (dist[edges[i].u] != INF && edges[i].w + dist[edges[i].u] < dist[edges[i].v]) {

11                 dist[edges[i].v] = edges[i].w + dist[edges[i].u];

12                 path[edges[i].v] = edges[i].u;

13             }

14         }

15     }

16 }

17 

18 //判断是否存在从原点可达的负权值回路

19 for (i = 0; i < m; ++i) {

20     if (dist[edges[i].u] != INF && edges[i].w + dist[edges[i].u] < dist[edges[i].v])

21         return 0;

22 }

23 return 1;

Bellman

 该算法还可以求最长路径

Bellman-Ford 判断环 + 最大路径值是否大于1

 1 #include<iostream>

 2 #include<cstdio>

 3 #include<cstdlib>

 4 #include<cstring>

 5 #include<string>

 6 #include<queue>

 7 #include<algorithm>

 8 #include<map>

 9 #include<iomanip>

10 #include<climits>

11 #include<string.h>

12 #include<cmath>

13 #include<stdlib.h>

14 #include<vector>

15 #include<set>

16 #define INF 1e7

17 #define MAXN 100010

18 #define maxn 50

19 #define maxm 1000

20 #define Mod 1000007

21 using namespace std;

22 typedef long long LL;

23 

24 struct exchange{

25     int ci, cj;

26     double cij;

27 }ex[maxm];

28 int i, j, k;

29 int n, m;

30 string name[maxn], a, b;

31 double x, maxdist[maxn];

32 bool flag;

33 int kase = 0;

34 

35 int readkase()

36 {

37     cin >> n;

38     if (n == 0) return 0;

39     for (i = 0; i < n; ++i)

40         cin >> name[i];

41     cin >> m;

42     for (i = 0; i < m; ++i) {

43         cin >> a >> x >> b;

44         for (j = 0; a != name[j]; ++j);

45         for (k = 0; b != name[k]; ++k);

46         ex[i].ci = j, ex[i].cj = k, ex[i].cij = x;

47     }

48     return 1;

49 }

50 

51 void Bellman(int v0)

52 {

53     flag = false;

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

55     maxdist[v0] = 1;

56     for (k = 1; k <= n; ++k) {   //从maxdist(0)递推到maxdist(n)

57         for (i = 0; i < m; ++i) {//判断加入每条边是否能是最大距离增大

58             if (maxdist[ex[i].ci] * ex[i].cij > maxdist[ex[i].cj])

59                 maxdist[ex[i].cj] = maxdist[ex[i].ci] * ex[i].cij;

60         }

61     }

62     if (maxdist[v0] > 1.0) flag = true;

63 }

64 

65 int main()

66 {

67     while (readkase()) {

68         for (int i = 0; i < n; ++i) {

69             Bellman(i);

70             if (flag) break;

71         }

72         if (flag) printf("Case %d: Yes\n",++kase);

73         else printf("Case %d: No\n", ++kase);

74     }

75     return 0;

76 }