These days Benny has designed a new compiler for C programming language. His compilation system provides a compiler driver that invokes the language preprocessor, compiler, assembler and linker. C source file (with .C suffix) is translated to relocatable object module first, and then all modules are linked together to generate an executable object file.
The translator (preprocessor, compiler and assembler) works perfectly and can generate well optimized assembler code from C source file. But the linker has a serious bug -- it cannot resolve global symbols when there are circular references. To be more specific, if file 1 references variables defined in file 2, file 2 references variables defined in file 3, ... file n-1 references variables defined in file n and file n references variables defined in file 1, then Benny's linker walks out because it doesn't know which file should be processed first.
Your job is to determine whether a source file can be compiled successfully by Benny's compiler.
Input
There are multiple test cases! In each test case, the first line contains one integer N, and then N lines follow. In each of these lines there are two integers Ai and Bi, meaning that file Ai references variables defined in file Bi (1 <= i <= N). The last line of the case contains one integer E, which is the file we want to compile.
A negative N denotes the end of input. Else you can assume 0 < N, Ai, Bi, E <= 100.
Output
There is just one line of output for each test case. If file E can be compiled successfully output "Yes", else output "No".
Sample Input
4
1 2
2 3
3 1
3 4
1
4
1 2
2 3
3 1
3 4
4
-1
Sample Output
No
Yes
Author: ZHENG, Lu
Source: Zhejiang Provincial Programming Contest 2005
//1859524 2009-05-08 09:52:20 Time Limit Exceeded 2475 C++ 1001 0 Wpl
//1859528 2009-05-08 09:56:35 Memory Limit Exceeded 2475 C++ 0 32769 Wpl
//1859534 2009-05-08 10:00:46 Wrong Answer 2475 C++ 0 224 Wpl
//1859641 2009-05-08 11:10:30 Accepted 2475 C++ 0 224 Wpl
#include <iostream>
#include <queue>
#define MAX 102
using namespace std;
int edges[MAX][MAX];
int n,start;
bool mark,used[MAX];
void Init()
{
int i,a,b,j;
for(i=1;i<MAX;i++)
for(j=1;j<MAX;j++)
edges[i][j]=-1;
for(i=0;i<n;i++)
{
scanf("%d%d",&a,&b);
if(a!=b)
edges[a][b]=1;
}
}
void DFS(int start)
{
int i;
if(used[start])
{
mark=true;
return;
}
used[start]=true;
for(i=1;i<MAX;i++)
{
if(edges[start][i]==1)
{
DFS(i);
}
}
used[start]=false;
}
int main()
{
int i;
while(scanf("%d",&n)!=EOF&&n>=0)
{
Init();
scanf("%d",&start);
mark=false;
for(i=1;i<MAX;i++)
used[i]=false;
DFS(start);
if(!mark)
printf("Yes\n");
else
printf("No\n");
}
return 0;
}
//邻接表
#include <iostream>
#include <vector>
#define MAX 102
using namespace std;
vector<int>G[MAX];
vector<int>::iterator p;
int n,start;
bool mark,used[MAX];
void Init()
{
int i;
int a,b;
for(i=1;i<MAX;i++)
{
G[i].clear();
used[i]=false;
}
for(i=1;i<=n;i++)
{
scanf("%d%d",&a,&b);
if(a!=b) //a a 或者b b这种不算是环的
G[a].push_back(b);
}
}
void DFS(int start)
{
if(used[start])
{
mark=true;
return;
}
used[start]=true;
/*if(G[start].empty())
return ;
for(p=G[start].begin();p!=G[start].end();p++)
DFS(*p);*/
int i;
for(i=0;i<G[start].size();i++)
DFS(G[start][i]);
used[start]=false;
}
int main()
{
while(scanf("%d",&n)!=EOF&&n>=0)
{
Init();
scanf("%d",&start);
mark=false;
DFS(start);
if(!mark)
printf("Yes\n");
else
printf("No\n");
}
return 0;
}