【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）

 #include <iostream>
 #include <cstdlib>
 #include <cstring>
 #include <cctype>
 #include <cmath>
 #include <string>
 #include <cstdio>
 #include <algorithm>
 #include <numeric>
 using namespace std;

 const int maxn = ;

 int  father[maxn];
 int  eage[maxn];
 bool vis[maxn], flag = ;
 int sum = ;

 int getFather (int x) {
     while (father[x] != x) {
         x = father[x];
     }
     return x;
 }

 void Union (int p, int q) {
     int x = getFather (p);
     int y = getFather (q);
     if (x != y) {
         father[y] = x;
         sum ++;
     } else {
         flag = ;
     }
 }

 int main () {
     int x, y, cur = ;
     while (cin >> x >> y) {
         if (x <  && y < ) break;
         if (x ==  && y == ) {
             printf("Case %d is a tree.\n", ++ cur);
             continue;
         } else {
             flag = ;
             memset(vis, , sizeof(vis));
             memset(eage, , sizeof(eage));
             for (int i = ; i < maxn; ++ i) {
                 father[i] = i;
             }
             vis[x] = vis[y] = ;
             Union(x, y);
             eage[y] ++;
             while (cin >> x >> y) {
                 if (x + y == ) break;
                 vis[x] = vis[y] = ;
                 Union(x, y);
                 eage[y] ++;
             }
             sort(eage, eage + maxn, greater<int>());
             int xx = ;
             if (eage[] > ) flag = ;
             for (int i = ; i < maxn; ++ i) {
                 if (vis[i] && father[i] == i) {
                     xx ++;
                     if (xx > ) {flag = ; break;}
                 }
             }
             /*for (int i = 1 ; i < maxn; ++ i) {
                 cout << vis[i] <<  " " ;
             }*/

             if (flag) printf("Case %d is a tree.\n", ++ cur);
             else printf("Case %d is not a tree.\n", ++ cur);
         }
     }
     return ;
 }