POJ 2553 —— The Bottom of a Graph

#include<cstdio>
#include<string>
#include<cstring>
#include<queue>
#include<vector>
#include<algorithm>
#define inf 0x3f3f3f3f
using namespace std;
const int maxn = 5005;

int n, m;
int DFN[maxn], Low[maxn], Stack[maxn], Belong[maxn];
bool Instack[maxn];
int Time, taj, top;
int head[maxn], edgenum;

struct Edge
{
int from, to, next;
}edge[maxn*maxn];

vector<int>bcc[maxn];
void tarjan(int u, int fa)
{
DFN[u] = Low[u] = ++Time;
Stack[top++] = u;
Instack[u] = true;

for(int i = head[u];i != -1;i = edge[i].next)
{
int v = edge[i].to ;
if(DFN[v] == -1)
{
tarjan(v, u) ;
Low[u] = min(Low[u], Low[v]);
}
else if(Instack[v] && Low[u] > DFN[v])
Low[u] = DFN[v];
}
if(Low[u] == DFN[u])
{
taj++;
bcc[taj].clear();
while(1)
{
int now = Stack[--top];
Instack[now] = false;
Belong[now] = taj;
bcc[taj].push_back(now);
if(now == u)	break;
}
}
}

void tarjan_init(int all)
{
memset(DFN, -1, sizeof DFN);
memset(Instack, false, sizeof Instack);
top = Time = taj = 0;
for(int i = 1;i<=all;i++)
{
if(DFN[i] == -1)
tarjan(i, i);
}
}

void add(int u, int v)
{
edge[edgenum].from = u;
edge[edgenum].to = v;
edge[edgenum].next = head[u];
head[u] = edgenum++;
}

vector<int>G[maxn];
int du[maxn];
void suodian()
{
for(int i = 1;i<=taj;i++)	G[i].clear();
memset(du, 0, sizeof du);
for(int i = 0;i<edgenum;i++)
{
int u = Belong[edge[i].from];
int v = Belong[edge[i].to];
if(u != v)
{
G[u].push_back(v);
du[u]++;
}
}
}

void init()
{
memset(head, -1, sizeof head);
edgenum = 0;
}

int main()
{
while(~scanf("%d", &n) && n)
{
scanf("%d", &m);
init();
while(m--)
{
int u, v;
scanf("%d%d", &u, &v);
add(u, v);
}
tarjan_init(n);
suodian();
int ans[maxn];
int cnt = 0;
for(int i = 1;i<=taj;i++)
{
if(du[i] == 0)
{
for(int j = 0;j<(int)bcc[i].size();j++)
{
int x = bcc[i][j];
ans[cnt++] = x;
}
}
}
sort(ans, ans+cnt);
for(int i = 0;i<cnt;i++)
{
if(i == cnt-1)
printf("%d\n", ans[i]);
else
printf("%d ", ans[i]);
}
}
return 0;
}