160. 相交链表

https://leetcode.cn/problems/intersection-of-two-linked-lists/
需要注意的点
1.链表a和b组合起来，如果有交点，

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    def getIntersectionNode(self, headA: ListNode, headB: ListNode) -> ListNode:
        if not headA or not headB:
            return None
        pa = headA
        pb = headB
        while pa != pb:
            if pa:
                pa = pa.next
            else:
                pa = headB
            if pb:
                pb = pb.next
            else:
                pb = headA
        return pa