poj 2762 Going from u to v or from v to u?

#include<iostream>
#include<cstring>
#include<cstdio>
#define N 1010
#define M 6010
using namespace std;
struct node{
int to,next;
}edge[M];
int head
,tot;
int dfn
,low
,stap
,belong
;
bool instack
,mat

;
int index,bcnt,top;
int dg
;
int n;
void init()
{
tot=0;
memset(head,-1,sizeof(head));
memset(dfn,0,sizeof(dfn));
memset(instack,false,sizeof(instack));
memset(mat,false,sizeof(mat));
index=top=bcnt=0;
}
void insert(int u,int v)
{
edge[tot].to=v;
edge[tot].next=head[u];
head[u]=tot++;
}
void tarjan(int u)
{
dfn[u]=low[u]=++index;
instack[u]=true;
stap[++top]=u;
for(int i=head[u];i!=-1;i=edge[i].next)
{
int v=edge[i].to;
if(!dfn[v])
{
tarjan(v);
low[u]=min(low[u],low[v]);
}
else if(instack[v]&&dfn[v]<low[u])
low[u]=dfn[v];
}
if(low[u]==dfn[u])
{
int x;
bcnt++;
do{
x=stap[top--];
instack[x]=false;
belong[x]=bcnt;
}while(x!=u);
}
}
void build()
{
for(int i=1;i<=n;i++)
for(int j=head[i];j!=-1;j=edge[j].next)
{
int v=edge[j].to;
mat[belong[i]][belong[v]]=true;
}
return;
}
/*
void show()
{
for(int i=1;i<=bcnt;i++)
{
for(int j=1;j<=bcnt;j++)
cout<<mat[i][j]<<" ";
cout<<endl;
}
}
*/
bool toposort()
{
int i,j,k,pos,del;
memset(dg,0,sizeof(dg));
for(i=1;i<=bcnt;i++)
{
for(j=1;j<=bcnt;j++)
if(i!=j)
dg[i]+=mat[j][i];
//cout<<dg[i]<<" ";
}
//cout<<endl;
for(i=1;i<=bcnt;i++)
{
pos=0;
del=0;
for(j=1;j<=bcnt;j++)
{
if(dg[j]==0)
{
dg[j]=-1;
del=j;
pos++;
}
if(pos>1) return false;
}
if(pos==0) return false;
for(k=1;k<=bcnt;k++)
dg[k]-=mat[del][k];
}
return true;
}
int main()
{
int t;
int i;
int u,v;
int m;
scanf("%d",&t);
while(t--)
{
init();
scanf("%d%d",&n,&m);
for(i=1;i<=m;i++)
{
scanf("%d%d",&u,&v);
insert(u,v);
}
for(i=1;i<=n;i++)
if(!dfn[i])
tarjan(i);
if(bcnt==1)
{
printf("Yes\n");
continue;
}
build();
//show();
if(toposort())
printf("Yes\n");
else
printf("No\n");
}
return 0;
}