[无向图割点] PKU 1523 SPF

targan算法。

 1 # include <cstdio>

 2 # include <cstring>

 3 

 4 # define N (1000 + 5)

 5 

 6 int n, son, tmpdfn;

 7 int low[N], dfn[N], subnets[N];

 8 char g[N][N];

 9 

10 int Min(int x, int y)

11 {

12     return x<y ? x:y;

13 }

14 

15 void tarjan(int u, int r)

16 {

17     dfn[u] = low[u] = ++tmpdfn;

18     for (int v = 1; v <= n; ++v) if (g[u][v])

19     {

20         if (!dfn[v])

21         {

22             //dfn[v] = low[v] = ++tmpdfn;  // 时间戳打在这里就忽略了节点r，原图可能是不联通的（这道题是联通的）

23             tarjan(v, r);

24             low[u] = Min(low[u], low[v]);

25             if (low[v] >= dfn[u])

26             {

27                 if (u != r) ++subnets[u];

28                 else ++son;

29             }

30         }

31         else

32             low[u] = Min(low[u], dfn[v]);

33     }

34 }

35 

36 void init(void)

37 {

38     tmpdfn = 0;

39     memset(low+1, 0, sizeof(int)*n);

40     memset(dfn+1, 0, sizeof(int)*n);

41     memset(subnets+1, 0, sizeof(int)*n);

42 }

43 

44 void solve(void)

45 {

46     bool find = false;

47     for (int i = 1; i <= n; ++i) if (!dfn[i])

48     {

49         son = 0;

50         tarjan(i, i);

51         if (son > 1) subnets[i] = son - 1;

52     }

53     for (int i = 1; i <= n; ++i)

54     {

55         if (subnets[i])

56         {

57             find = true;

58             printf("\n  SPF node %d leaves %d subnets", i, subnets[i]+1);

59         }

60     }

61     if (find == false)

62         printf("\n  No SPF nodes");

63 }

64 

65 int read_graph(void)

66 {

67     n = 0;

68     memset(g, 0, sizeof(g));

69     int x, y;

70     scanf("%d", &x);

71     if (x == 0) return 0;

72     do

73     {

74         scanf("%d", &y);

75         g[x][y] = g[y][x] = 1;

76         if (x > n) n = x;

77         if (y > n) n = y;

78     }while (scanf("%d", &x), x);    

79     return 1;

80 }

81 

82 int main()

83 {    

84     int icase = 0;

85     while (read_graph())

86     {

87         if (icase != 0) printf("\n\n");

88         printf("Network #%d", ++icase);

89         init();

90         solve();

91     }

92 }

注意n=1的情况。