hdu 3342(利用拓扑排序过程判断有向图是否有回路及自环)

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/*

  Name: 利用拓扑排序过程判断有向图是否有回路及自环 

  Copyright: 

  Author: Try86

  Date: 15/04/12 21:15

  Description: 

*/



#include <cstdio>

#include <cstdlib>

#include <cstring>

#include <iostream>



using namespace std;



const int N = 105;



int color[N], flag;

struct node {

    int v;

    node *next;

    node(int vv, node *p) {

        v = vv;

        next = p;

    }

};

struct graph {

    node *link;

}G[N];



void init(int n) {

    for (int i=0; i<n; ++i) G[i].link = NULL;

    return ;

}



void buildG(int u, int v) {

    node *p = new node(v, G[u].link);

    G[u].link = p;

    return ;

}



void dfsVisit(int u) {

    color[u] = 1;  

    for (node *p=G[u].link; p; p=p->next) {

        if (color[p->v] == 1) {//发现回路 

            flag = false;

            return ;

        }

        if (!color[p->v]) dfsVisit(p->v);

    }

    color[u] = 2;

    return ;

}



void topoSort(int n) {

    flag = true;

    memset(color, 0, sizeof(color));

    for (int i=0; i<n; ++i) {

        if (!color[i]) {

            dfsVisit(i);

            if (!flag) return ;

        }

    }

    return ;

}



void del(node *p) {

    if (!p) return ;

    del(p->next);

    delete p;

}



int main() {

    int n, m;

    while (scanf("%d%d", &n, &m), n+m) {

        init(n);

        int u, v;

        for (int i=0; i<m; ++i) {

            scanf ("%d%d", &u, &v);

            buildG(u, v);

        }

        topoSort(n);

        if (flag) printf ("YES\n");

        else printf ("NO\n");

        for (int i=0; i<n; ++i) del(G[i].link);

    }

    return 0;

}