python判断无向图环是否存在的示例

#无向图判断环是否存在
def dfs(u,fa):
  for i in range(v):
    n=g[u][i]#n为图中的顶点数
    # print(u,n,fa,i,'')
    if n in vertex:#判断n是否属于图的顶点
      if n==fa:
        continue
      if visit[n]==0:
        visit[n]=1
        if dfs(n,u)==1:
          return 1
      else:
        return 1
  return 0
global flag,visit,v,g,e,vertex
vertex=[]
flag=0
v=4;e=4#v是顶点，e是边数
#v,e=map(int,input().split())
vertex=[1,2,3,4]#无向图的顶点集合
g=[[0 for i in range(20)]for i in range(20)]
visit=[0 for i in range(20)]
g[1][0]=2;g[2][0]=1;g[2][1]=3;g[3][0]=2;g[3][1]=4;g[4][0]=3;g[1][1]=3;g[3][2]=1
for i in range(1,v+1):
  if visit[i]==0:
    visit[i]=1
    if dfs(i,-1):
      flag=1
      break
if flag==1:
  print('Yes, there is A loop in this network')
else:
  print('No loop')