1 //只需要判断入度为零的点是不是只有1个就可以了
2 #include<stdio.h>
3 #include<string.h>
4 #include<string>
5 #include<algorithm>
6 #include<map>
7 using namespace std;
8 #define maxn 1005
9 int indegree[maxn];
10 map<string,int>re;
11 void init()
12 {
13 re.clear();
14 memset(indegree,0,sizeof(indegree));
15 }
16 void topsort(int num)
17 {
18 int count,i;
19 count=0;
20 for(i=1;i<=num;i++)
21 if(indegree[i]==0)
22 count++;
23 if(count==1)
24 printf("Yes\n");
25 else
26 printf("No\n");
27 }
28 int main()
29 {
30 char a[20],b[20];
31 int n,i,j,people;
32 while(scanf("%d",&n)!=EOF&&n)
33 {
34 init();
35 people=1;
36 for(i=1;i<=n;i++)
37 {
38 scanf("%s%s",a,b);
39 if(re[a]==0)
40 re[a]=people++;
41 if(re[b]==0)
42 re[b]=people++;
43 indegree[re[b]]++;
44 }
45 topsort(people-1);
46 }
47 return 0;
48 }
