Explain if the runtime of the function below is O(len(L))

def isTrail(L,n):

p = True

M = {}

for i in range(len(n)-1):

if(L[n[i]][n[i+1]] ==0 or (n[i],n[i+1]) in M or (n[i+1],n[i]) in M):

return False

else:

M[(n[i],n[i+1])] = 1

return p