【HDU1325】Is It A Tree?（并查集基础题）

有以下坑点：

1.结束输入不一定-1，题目中的叙述只是说所有权值都为正值。

2.是否构成一棵树不能只判断是否只有一个根节点，没有环路，而且还需要判断每个节点的入度一定是1，不然就不是一棵树。

（无环路也可用树的性质：结点数 = 边树 + 1 来取代）

 

 1 #include <iostream>

 2 #include <cstdlib>

 3 #include <cstring>

 4 #include <cctype>

 5 #include <cmath>

 6 #include <string>

 7 #include <cstdio>

 8 #include <algorithm>

 9 #include <numeric>

10 using namespace std;

11 

12 const int maxn = 25;

13 

14 int  father[maxn];

15 int  eage[maxn];

16 bool vis[maxn], flag = 0;

17 int sum = 0;

18 

19 int getFather (int x) {

20     while (father[x] != x) {

21         x = father[x];

22     }

23     return x;

24 }

25 

26 void Union (int p, int q) {

27     int x = getFather (p);

28     int y = getFather (q);

29     if (x != y) {

30         father[y] = x;

31         sum ++;

32     } else {

33         flag = 0;

34     }

35 }

36 

37 int main () {

38     int x, y, cur = 0;

39     while (cin >> x >> y) {

40         if (x < 0 && y < 0) break;

41         if (x == 0 && y == 0) {

42             printf("Case %d is a tree.\n", ++ cur);

43             continue;

44         } else {

45             flag = 1;

46             memset(vis, 0, sizeof(vis));

47             memset(eage, 0, sizeof(eage));

48             for (int i = 0; i < maxn; ++ i) {

49                 father[i] = i;

50             }

51             vis[x] = vis[y] = 1;

52             Union(x, y);

53             eage[y] ++;

54             while (cin >> x >> y) {

55                 if (x + y == 0) break;

56                 vis[x] = vis[y] = 1;

57                 Union(x, y);

58                 eage[y] ++;

59             }

60             sort(eage, eage + maxn, greater<int>());

61             int xx = 0;

62             if (eage[0] > 1) flag = 0;

63             for (int i = 1; i < maxn; ++ i) {

64                 if (vis[i] && father[i] == i) {

65                     xx ++;

66                     if (xx > 1) {flag = 0; break;}

67                 }

68             }

69             /*for (int i = 1 ; i < maxn; ++ i) {

70                 cout << vis[i] <<  " " ;

71             }*/

72 

73             if (flag) printf("Case %d is a tree.\n", ++ cur);

74             else printf("Case %d is not a tree.\n", ++ cur);

75         }

76     }

77     return 0;

78 }