hdu2094(拓扑排序)

/*****************************************
Author      :Crazy_AC(JamesQi)
Time        :2015
File Name   :
*****************************************/
// #pragma comment(linker, "/STACK:1024000000,1024000000")
#include <iostream>
#include <algorithm>
#include <iomanip>
#include <sstream>
#include <string>
#include <stack>
#include <queue>
#include <deque>
#include <vector>
#include <map>
#include <set>
#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>
#include <limits.h>
using namespace std;
#define MEM(a,b) memset(a,b,sizeof a)
typedef long long LL;
typedef unsigned long long ULL;
typedef pair<int,int> ii;
const int inf = 1 << 30;
const int INF = 0x3f3f3f3f;
const int MOD = 1e9 + 7;
int n;
int in[1010];
map<string,int> mp;
int main()
{
// freopen("in.txt","r",stdin);
// freopen("out.txt","w",stdout);
while(~scanf("%d",&n) && n){
// cout << "n = " << n << endl;
string name1,name2;
mp.clear();
MEM(in, 0);
int tot = 1;
for (int i = 1;i <= n;i++){
cin >> name1 >> name2;
if (!mp[name1]) mp[name1] = tot++;
if (!mp[name2]) mp[name2] = tot++;
in[mp[name2]]++;
}
int cnt = 0;
for (int i = 1;i < tot;i++){
if (in[i] == 0) cnt++;
if (cnt > 1) break;
}
if (cnt == 1) printf("Yes\n");
else printf("No\n");
}
return 0;
}