【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 }

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