HDU 1325 拓扑排序

 1 #include <cstdio>
 2 #include <cstring>
 3 #include <iostream>
 4 using namespace std;
 5 
 6 const int N = 100005;
 7 int in[N] , first[N] , k , maxn , del , sum;//maxn记录总共的点的个数，del记录删除了的点的个数
 8 
 9 struct Edge{
10     int y , next;
11 }e[N<<2];
12 
13 void add_edge(int x , int y )
14 {
15     e[k].y = y , e[k].next = first[x];
16     first[x] = k++;
17 }
18 
19 void tuopu(int src)
20 {
21     del++;
22     for(int i = first[src] ; i!=-1 ; i=e[i].next){
23         int v = e[i].y;
24         in[v]--;
25         if(!in[v]) tuopu(v);
26     }
27 }
28 
29 int main()
30 {
31    // freopen("a.in" , "r" , stdin);
32     int x , y , cas = 0;
33     while(~scanf("%d%d" , &x , &y)){
34         if(x < 0 && y < 0) break;
35 
36         memset(first , -1 , sizeof(first));
37         memset(in , -1 , sizeof(in));
38         k = 0 , maxn = 0 , sum = 0;//sum表示总共点的个数
39         int  cnt = 0 , root , flag = 0; //cnt记录有多少节点可以作为顶点了,flag作为判断是否有点超过1个入度
40         while(x != 0 || y!=0){
41             add_edge(x , y);
42             maxn = max(max(maxn , x) , y);
43             if(in[x] < 0) in[x] = 0 , sum++;
44             if(in[y] < 0) in[y] = 0 , sum++;
45             in[y]++;
46             if(in[y] >= 2){
47                 flag = 1;
48             }
49             scanf("%d%d" , &x , &y);
50         }
51        // cout<<"maxn: "<<maxn<<endl;
52         if(flag){
53           //  cout<<"有点超过两个入度 "<<endl;
54             printf("Case %d is not a tree.\n" , ++cas);
55             continue;
56         }
57         for(int i = 1 ; i<=maxn ; i++)
58             if(!in[i]){
59                 root = i;
60                 cnt++;
61                 if(cnt>=2) break;
62             }
63         if(cnt >= 2){
64            // cout<<"有超过两个点可作为树根 "<<endl;
65             printf("Case %d is not a tree.\n" , ++cas);
66             continue;
67         }
68         del = 0;
69         tuopu(root);
70         if(del < sum)
71         {
72            // cout<<"出现内部环 "<<endl;
73             printf("Case %d is not a tree.\n" , ++cas);
74         }
75         else
76             printf("Case %d is a tree.\n" , ++cas);
77     }
78     return 0;
79 }