1. 方法一：哈希集合 判断是否存在公共节点

# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

#
# 
# @param pHead1 ListNode类 
# @param pHead2 ListNode类 
# @return ListNode类
#
class Solution:
    def FindFirstCommonNode(self , pHead1 , pHead2 ):
        # write code here
        if(pHead1==None)|(pHead2==None):
            return None
        node1,node2 = pHead1,pHead2
        set_A = set()
        while node1:
            set_A.add(node1)
            node1=node1.next
        while node2:
            if node2 in set_A:
                return node2
            node2 = node2.next
        return None
        

1. 记住哈希集合的使用方法：set_A = set()
2. 哈希集合判是否存在方法： if node2 in set_A
3. Python里面是 if-else-elif 不是 else if
4. 这样的空间复杂度比较高，但是相较于数组存储的话，查找效率很高

# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

#
# 
# @param pHead1 ListNode类 
# @param pHead2 ListNode类 
# @return ListNode类
#
class Solution:
    def FindFirstCommonNode(self , pHead1 , pHead2 ):
        # write code here
        if(pHead1==None)|(pHead2==None):
            return None
        p1,p2= pHead1,pHead2
        while p1!=p2:
            if p1:
                p1=p1.next
            else:
                p1 = pHead2
            if p2:
                p2 = p2.next
            else:
                p2 = pHead1
        if p1:
            return p1
        else:
            return None
        

双指针算法，一图以言之，两个指针都走了 a+b+c = c + b + a 的路程