On LeetCode, why do "lists" behave like objects, with a `.val` and `.next` property?
Question:
https://leetcode.com/problems/add-two-numbers/editorial/
On the leetcode website, there is a "Add Two Number" practice. My questions is, how does l1.val or l2.val work? I thought l1 is a list which is [2, 4, 3]. How does l1.val calling integer 2?
Or it’s from class ListNode? I’m confused..
Code solution as below:
Definition for singly-linked list.
class ListNode:
def __init__(self, val=0, next=None):
self.val = val
self.next = next
class Solution:
def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
dummyHead = ListNode(0)
curr = dummyHead
carry = 0
while l1 != None or l2 != None or carry != 0:
l1Val = l1.val if l1 else 0
l2Val = l2.val if l2 else 0
columnSum = l1Val + l2Val + carry
carry = columnSum // 10
newNode = ListNode(columnSum % 10)
curr.next = newNode
curr = newNode
l1 = l1.next if l1 else None
l2 = l2.next if l2 else None
return dummyHead.next
There are l1 = [2, 4, 3] and l2 = [5, 6, 4]. I thought to access the integer inside the list, you need to write l1[index]?
Why the code is using l1.val?
Answers:
For brevity, they are written as lists, but they are not
If they really were simply list objects, you would be able to access the elements as l1[0] etc.
However they are in fact ListNode objects.
l1 is, in effect,
{
"val":2,
"next": {
"val":4,
"next": {
"val":3,
"next":None
}
}
}
They are just writing [2,4,3] as a shorthand for the above structure. I agree that it is confusing.
l1 and l2 are specified to be of type ListNode when defined
def addTwoNumbers(self, l1: Optional[ListNode], ...
##################
The code above means, "If parameter l1 is present, it is of type ListNode".
The definition of ListNode is given in the previous block, beginning class ListNode:.
In other words, class Solution depends on class ListNode.
https://leetcode.com/problems/add-two-numbers/editorial/
On the leetcode website, there is a "Add Two Number" practice. My questions is, how does l1.val or l2.val work? I thought l1 is a list which is [2, 4, 3]. How does l1.val calling integer 2?
Or it’s from class ListNode? I’m confused..
Code solution as below:
Definition for singly-linked list.
class ListNode:
def __init__(self, val=0, next=None):
self.val = val
self.next = next
class Solution:
def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
dummyHead = ListNode(0)
curr = dummyHead
carry = 0
while l1 != None or l2 != None or carry != 0:
l1Val = l1.val if l1 else 0
l2Val = l2.val if l2 else 0
columnSum = l1Val + l2Val + carry
carry = columnSum // 10
newNode = ListNode(columnSum % 10)
curr.next = newNode
curr = newNode
l1 = l1.next if l1 else None
l2 = l2.next if l2 else None
return dummyHead.next
There are l1 = [2, 4, 3] and l2 = [5, 6, 4]. I thought to access the integer inside the list, you need to write l1[index]?
Why the code is using l1.val?
For brevity, they are written as lists, but they are not
If they really were simply list objects, you would be able to access the elements as l1[0] etc.
However they are in fact ListNode objects.
l1 is, in effect,
{
"val":2,
"next": {
"val":4,
"next": {
"val":3,
"next":None
}
}
}
They are just writing [2,4,3] as a shorthand for the above structure. I agree that it is confusing.
l1 and l2 are specified to be of type ListNode when defined
def addTwoNumbers(self, l1: Optional[ListNode], ...
##################
The code above means, "If parameter l1 is present, it is of type ListNode".
The definition of ListNode is given in the previous block, beginning class ListNode:.
In other words, class Solution depends on class ListNode.