Topological sorting (learning notes) introduction + judge whether there is a ring

A topological sort （ Learning notes ）

For a directed acyclic graph （DAG） Topological sort .
Realization ：
1. Select a vertex in a directed graph without a precursor and output
2. Delete the vertex and all outgoing edges from the graph
3. repeat 1、2 step , Until all vertices output
initialization ：

vector<int>vec[maxn];
scanf("%d%d",&n,&m);//n A little bit ,m side 
int n,m,u,v;
int InDeg[maxn];// The degree of  
while(m--){
    
	scanf("%d%d",&u,&v);
	vec[u].push_back(v);
	InDeg[v]++;
}

Judge whether there is a ring ：

queue<int>q; 
int num;// Count the number of ordered elements  
bool topsort(){
    
	for(int i=1;i<=n;i++){
    
		if(!InDeg[i]) q.push(i);
	}
	while(!q.empty()){
    
		int now=q.front();
		q.pop();
		num++;
		for(int i=0;i<vec[now].size();i++){
    
			if(--InDeg[vec[now][i]]==0)
				q.push(vec[now][i]);
		}
	}
	if(num==n) return true;// acyclic  
	else return false;// Ring  
}