hdu 1269 (强联通分量Tarjan入门)

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#define ll long long
using namespace std;
const int MAXN = 12000;
const int MAXM = 120000;
struct Edge
{
int to,next;
}edge[MAXM];
int head[MAXN],tot;
int Low[MAXN],DFN[MAXN],Stack[MAXN],Belong[MAXN];
int Index,top;
int scc;
bool Instack[MAXN];
int num[MAXN];

void addedge(int u,int v)
{
edge[tot].to = v;edge[tot].next = head[u];head[u] = tot++;
}
void Tarjan(int u)
{
int v;
Low[u] = DFN[u] = ++Index;
Stack[top++] = u;
Instack[u] = true;
for(int i = head[u]; i != -1; i = edge[i].next)
{
v = edge[i].to;
if( !DFN[v] )
{
Tarjan(v);
if(Low[u] > Low[v]) Low[u] = Low[v];
}
else if(Instack[v] && Low[u] >  DFN[v])
Low[u] = DFN[v];
}
if(Low[u] == DFN[u])
{
scc++;
do
{
v = Stack[--top];
Instack[v] = false;
Belong[v] = scc;
num[scc]++;
}
while(v != u);
}
}
void solve(int N)
{
memset(DFN,0,sizeof(DFN));
memset(Instack,false,sizeof(Instack));
memset(num,0,sizeof(num));
Index = scc = top = 0;
for(int i = 1; i <= N; i++)
if( !DFN[i])
Tarjan(i);
}
void init()
{
tot = 0;
memset(head,-1,sizeof(head));
}
int main(void)
{
int n,m,a,b;
while(scanf("%d %d",&n,&m) ,n != 0 || m != 0)
{
init();
for(int i = 0; i < m; i++)
{
scanf("%d %d",&a,&b);
addedge(a,b);
}
solve(n);
if(scc == 1)
printf("Yes\n");
else
printf("No\n");
}
return 0;
}