HDU.3342 Legal or Not (拓扑排序 TopSort)

题意分析

裸的拓扑排序 根据是否成环来判断是否合法

详解请移步

算法学习 拓扑排序（TopSort）

代码总览

#include <iostream>

#include <iostream>

#include <cstdio>

#include <algorithm>

#include <cstring>

#include <sstream>

#include <set>

#include <map>

#include <queue>

#include <stack>

#include <cmath>

#define nmax 200

#define MEM(x) memset(x,0,sizeof(x))

using namespace std;

int n,m;

int indegree[nmax];

vector<int> v[nmax];

bool suc = true;

void topsort()

{

    int cnt = 0;

    queue<int> q;

    while(1){

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

            if(indegree[i] == 0){

                q.push(i);

                cnt++;

                indegree[i] = -1;

            }

        }

        if(q.empty()) break;

        while(!q.empty()){

            int t = q.front();q.pop();

            for(int i = 0; i<v[t].size();++i){

                int tt = v[t][i];

                if(indegree[tt] == -1){

                    suc =false;

                    break;

                }else

                    indegree[tt]--;

            }

            if(!suc) break;

        }

        if(!suc) break;

    }

    if(suc && cnt!=n) suc = false;

}

int main()

{

    //freopen("in.txt","r",stdin);

    while(scanf("%d%d",&n,&m) == 2 && n){

        MEM(indegree);

        suc = true;

        for(int i = 0; i<n;++i) v[i].clear();

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

            int a,b;

            scanf("%d%d",&a,&b);

            indegree[b]++;

            v[a].push_back(b);

        }

        topsort();

        printf("%s\n",suc?"YES":"NO");

    }

    return 0;

}