1 #include<stdio.h>
 2 #include<string.h>
 3 int main()
 4 {
 5     int compair(char a[],char b[]);
 6     int n,i,j,t,frist;
 7     struct
 8     {
 9         char a[100];
10         char b[100];
11     } grade[1000];
12     while(~scanf("%d",&n)&&n)
13     {
14         memset(grade,0,sizeof grade);
15         for(i=0; i<n; ++i)
16             scanf("%s%s",grade[i].a,grade[i].b);
17         for(i=t=0; i<n; ++i)
18         {
19             for(j=0; j<n; ++j)
20                 if(compair(grade[i].a,grade[j].b))
21                     break;
22             if(j==n)
23                 if(t)
24                 {
25                     if(!compair(grade[frist].a,grade[i].a))
26                         ++t;
27                 }
28                 else
29                 {
30                     frist=i;
31                     ++t;
32                 }
33             //printf("t=%d\n",t);
34             if(t>1)
35             {
36                 printf("No\n");
37                 break;
38             }
39         }
40         if(t==1)
41             printf("Yes\n");
42         else if(t<1)
43             printf("No\n");
44     }
45 }
46 
47 int compair(char a[],char b[])
48 {
49     int i;
50     for(i=0; a[i]!=‘\0‘&&b[i]!=‘\0‘; ++i)
51         if(a[i]!=b[i])
52         {
53             return 0;
54             break;
55         }
56     if(a[i]!=‘\0‘||b[i]!=‘\0‘)
57         return 0;
58     else
59         return 1;
60 }