POJ 2553 The Bottom of a Graph "SCC Tarjan"

The strong connecting review of graph theory begins--

The main topic: give you a map, you want to find such a point set: From this point to get to the point, must be able to return to this point

Idea: The strong connected components in the obvious can get to each other, then consider, after the contraction point if a point has an edge, must not be in the point set, because the shrinking point is a dag, no ring, so must not go back to the original point, so find out the degree of 0 points can be

#include <cstdio>

#include <string.h>

#include <math.h>

#include <algorithm>

#include <iostream>

#include <queue>

#define MAXN 90000

#define INF 0x3f3f3f3f

using namespace Std;

int head[maxn],next[maxn],point[maxn],now=0;

int DFN[MAXN],LOW[MAXN],TIME,COL,STACK[MAXN];

int TOP,BELONG[MAXN],OUT[MAXN];

BOOL INSTACK[MAXN];

void Add (int x,int y)

{

NEXT[++NOW]=HEAD[X];

Head[x]=now;

Point[now]=y;

}

void Tarjan (int k)

{

int u;

Dfn[k]=low[k]=++time;

Instack[k]=1;

Stack[++top]=k;

for (int i=head[k];i;i=next[i])

{

U=point[i];

if (dfn[u]==0)

{

Tarjan (U);

Low[k]=min (Low[u],low[k]);

}

else if (Instack[u]) low[k]=min (Low[k],low[u]);

}

if (Low[k]==dfn[k])

{

++col;

Do

{

u=stack[top--];

instack[u]=0;

Belong[u]=col;

}while (U!=K);

}

}

int main ()

{

int n,m,x,y;

while (1)

{

scanf ("%d", &n);

if (n==0) break;

scanf ("%d", &m);

Now=0;memset (head,0,sizeof (head));

Top=0;memset (instack,0,sizeof (instack));

Memset (out,0,sizeof (out));

memset (dfn,0,sizeof (DFN));

for (int i=1;i<=m;i++)

{

scanf ("%d%d", &x,&y);

Add (x, y);

}

for (int i=1;i<=n;i++) if (dfn[i]==0) Tarjan (i);

for (int i=1;i<=n;i++)

{

for (int j=head[i];j;j=next[j])

{

int U=POINT[J];

if (Belong[i]!=belong[u]) out[belong[i]]++;

}

}

int flag=1;

for (int i=1;i<=n;i++)

{

if (flag && out[belong[i]]==0)

{

printf ("%d", I);

Flag^=flag;

}

else if (out[belong[i]]==0) printf ("%d", I);

}

printf ("\ n");

}

return 0;

}

POJ 2553 The Bottom of a Graph "SCC Tarjan"