Design a linear time algorithm for each of the following problem: (1) given a graph G, test if G is connected. (2) given a graph G, test if G is a tree. (3) given a graph G, test if G has a cycle,if so,print a cycle
实例如下：
import numpy
from numpy import *
def dfs( v ):
vis[v] = -1
flag = 0
for i in range(n):
# print (a[v][i],'---', vis[i] )
if a[v][i] != 0 and vis[i] != -1:
dfs(i)
vis[i] = 1
else:
pass
if a[v][i] != 0 and vis[i] == -1:
print ('Yes
剑指Offer(Python多种思路实现):环的入口节点
面试23题：
题目：如果一个链表中包含环，如何找出环的入口节点？
解题分析：其实此题可以分解为三个题目：1）如何判断一个链表中是否包含环？2）如何找到环的入口节点？3）如何得到环中节点的数目？
解题思路一：可以设置两个指针，一快一慢。
def detectCycle(self, head):
fast = slow = head
# 检测是否有环
while fast and fast.next:
