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 = 5005;
    const static int MAXM = MAXN * MAXN;
private:
    struct Edge{
        int to, next;
    }edge[MAXM];
    struct Point{
        int dfn, low, color;
        Point(){dfn = low = color = 0;}
    }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 != -1; 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 = 0;
        sumOfPoint = n;
        for(int i = 1; i <= n; i ++){
            first[i] = -1;
            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 = 1; i <= sumOfPoint; i ++){
            if(!point[i].dfn){
                tarjan(i);
            }
        }
    }
    vector<int> getAns(){
        vector<int> ans;
        int *outdegree = new int[sumOfPoint+1];
        for(int i = 1; i <= sumOfPoint; i ++){
            outdegree[i] = 0;
        }
        tarjanAllPoint();
        for(int i = 1; i <= sumOfPoint; i ++){
            for(int j = first[i]; j != -1; j = edge[j].next){
                int to = edge[j].to;
                if(point[to].color != point[i].color){
                    outdegree[point[i].color] ++;
                }
            }
        }
        for(int i = 1; i <= colorNum; i ++){
            if(!outdegree[i]){
                for(int j = 0; 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 = 1;
        for(int i = 0; i < ans.size(); i ++){
            if(first) first = 0;
            else putchar(' ');
            printf("%d", ans[i]);
        }
        putchar('
');
    }
    return 0;
}