题目地址:http://poj.org/problem?id=2553

The Bottom of a Graph
Time Limit: 3000MS   Memory Limit: 65536K
Total Submissions: 7881   Accepted: 3263

Description

We will use the following (standard) definitions from graph theory. Let V be a nonempty and finite set, its elements being called vertices (or nodes). Let E be a subset of the Cartesian product V×V, its elements being called edges. Then G=(V,E) is called a directed graph.  Let n be a positive integer, and let p=(e1,...,en) be a sequence of length n of edges ei∈E such that ei=(vi,vi+1) for a sequence of vertices (v1,...,vn+1). Then p is called a path from vertex v1 to vertex vn+1 in G and we say that vn+1 is reachable from v1, writing (v1→vn+1).  Here are some new definitions. A node v in a graph G=(V,E) is called a sink, if for every node w in G that is reachable from vv is also reachable from w. The bottom of a graph is the subset of all nodes that are sinks, i.e.,bottom(G)={v∈V|∀w∈V:(v→w)⇒(w→v)}. You have to calculate the bottom of certain graphs.

Input

The input contains several test cases, each of which corresponds to a directed graph G. Each test case starts with an integer number v, denoting the number of vertices of G=(V,E), where the vertices will be identified by the integer numbers in the set V={1,...,v}. You may assume that 1<=v<=5000. That is followed by a non-negative integer e and, thereafter, e pairs of vertex identifiers v1,w1,...,ve,we with the meaning that (vi,wi)∈E. There are no edges other than specified by these pairs. The last test case is followed by a zero.

Output

For each test case output the bottom of the specified graph on a single line. To this end, print the numbers of all nodes that are sinks in sorted order separated by a single space character. If the bottom is empty, print an empty line.

Sample Input

3 3
1 3 2 3 3 1
2 1
1 2
0

Sample Output

1 3
2

Source

 
【题解】:
  找到入度为0的所有强连通分量,将其中的点排序后输出
  这题跟poj 2186 很类似
  
【code】:
  

 /**
Judge Status:Accepted Memory:1488K
Time:125MS Language:G++
Code Length:2443B Author:cj
*/
#include<iostream>
#include<stdio.h>
#include<string.h>
#include<stack>
#include<vector>
#include<algorithm> #define N 10010
using namespace std; vector<int> G[N];
stack<int> stk;
int pre[N],lowlink[N],sccno[N],scc_cnt,dfn_clock,out[N],counter[N]; void DFN(int u) //tarjan算法
{
lowlink[u] = pre[u] = ++dfn_clock;
stk.push(u);
int i;
for(i=;i<G[u].size();i++)
{
int v = G[u][i];
if(!pre[v])
{
DFN(v);
lowlink[u] = min(lowlink[u],lowlink[v]);
}
else if(!sccno[v])
{
lowlink[u] = min(lowlink[u],pre[v]);
}
}
if(lowlink[u]==pre[u])
{
scc_cnt++; //强连通图的个数标记
while()
{
int x = stk.top();
stk.pop();
sccno[x] = scc_cnt;
if(x==u) break;
}
}
} void findscc(int n)
{
int i;
scc_cnt = dfn_clock = ;
memset(pre,,sizeof(pre));
memset(lowlink,,sizeof(lowlink));
memset(sccno,,sizeof(sccno));
for(i=;i<=n;i++)
if(!pre[i])
DFN(i);
} int main()
{
int n,m;
while(~scanf("%d",&n)&&n)
{
scanf("%d",&m);
int i;
for(i=;i<=n;i++)
G[i].clear();
for(i=;i<m;i++)
{
int a,b;
scanf("%d%d",&a,&b);
G[a].push_back(b); // 得到图
}
findscc(n); //查找强连通图
int j;
memset(out,,sizeof(out));
memset(counter,,sizeof(counter)); for(i=;i<=n;i++) //遍历一边图,查找统计个点缩点后的出度
{
for(j=;j<G[i].size();j++)
{
int v = G[i][j];
if(sccno[i]!=sccno[v])
{
out[sccno[i]]++; //出度
}
}
}
for(i=;i<=scc_cnt;i++)
{
if(!out[i]) //出度为0的强连通分量
{
counter[i] = ; //标记
}
} int pl = ;
for(i=;i<=n;i++)
if(counter[sccno[i]]) //是否被标记,从下到大
{
if(pl) printf(" %d",i);
else printf("%d",i);
pl = ;
}
putchar();
}
return ;
}

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