poj 2553 The Bottom of a Graph ： tarjan O(n) 存环中的点

 /**
 problem: http://poj.org/problem?id=2553
 将所有出度为0环中的点排序输出即可。
 **/

 #include<stdio.h>
 #include<stack>
 #include<vector>
 #include<algorithm>
 using namespace std;

 class Graphics{
     const static int MAXN = ;
     const static int MAXM = MAXN * MAXN;
 private:
     struct Edge{
         int to, next;
     }edge[MAXM];
     struct Point{
         int dfn, low, color;
         Point(){dfn = low = color = ;}
     }point[MAXN], emptyPoint;
     int first[MAXN], sign, colorNum, dfnNum, sumOfPoint;
     bool vis[MAXN];
     vector<int> ring[MAXN];
     stack<int> stk;
     void tarjan(int u){
         point[u].dfn = ++ dfnNum;
         point[u].low = dfnNum;
         vis[u] = true;
         stk.push(u);
         for(int i = first[u]; i != -; i = edge[i].next){
             int to = edge[i].to;
             if(!point[to].dfn){
                 tarjan(to);
                 point[u].low = min(point[to].low, point[u].low);
             }else if(vis[to]){
                 point[u].low = min(point[to].dfn, point[u].low);
             }
         }
         if(point[u].low == point[u].dfn){
             vis[u] = false;
             point[u].color = ++colorNum;
             ring[colorNum].push_back(u);
             while(stk.top() != u){
                 vis[stk.top()] = false;
                 point[stk.top()].color = colorNum;
                 ring[colorNum].push_back(stk.top());
                 stk.pop();
             }
             stk.pop();
         }
     }
 public:
     void clear(int n){
         sign = colorNum = dfnNum = ;
         sumOfPoint = n;
         for(int i = ; i <= n; i ++){
             first[i] = -;
             vis[i] = false;
             ring[i].clear();
             point[i] = emptyPoint;
         }
         while(!stk.empty()) stk.pop();
     }
     void addEdgeOneWay(int u, int v){
         edge[sign].to = v;
         edge[sign].next = first[u];
         first[u] = sign ++;
     }
     void tarjanAllPoint(){
         for(int i = ; i <= sumOfPoint; i ++){
             if(!point[i].dfn){
                 tarjan(i);
             }
         }
     }
     vector<int> getAns(){
         vector<int> ans;
         int *outdegree = new int[sumOfPoint+];
         for(int i = ; i <= sumOfPoint; i ++){
             outdegree[i] = ;
         }
         tarjanAllPoint();
         for(int i = ; i <= sumOfPoint; i ++){
             for(int j = first[i]; j != -; j = edge[j].next){
                 int to = edge[j].to;
                 if(point[to].color != point[i].color){
                     outdegree[point[i].color] ++;
                 }
             }
         }
         for(int i = ; i <= colorNum; i ++){
             if(!outdegree[i]){
                 for(int j = ; j < ring[i].size(); j ++){
                     ans.push_back(ring[i][j]);
                 }
             }
         }
         sort(ans.begin(), ans.end());
         delete []outdegree;
         return ans;
     }
 }graph;

 int main(){
     int n, m;
     while(scanf("%d%d", &n, &m) != EOF && n){
         graph.clear(n);
         while(m --){
             int a, b;
             scanf("%d%d", &a, &b);
             graph.addEdgeOneWay(a, b);
         }
         vector<int> ans = graph.getAns();
         bool first = ;
         for(int i = ; i < ans.size(); i ++){
             if(first) first = ;
             else putchar(' ');
             printf("%d", ans[i]);
         }
         putchar('\n');
     }
     return ;
 }