2130. Maximum Twin Sum Of A Linked List - Explanation

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def pairSum(self, head: Optional[ListNode]) -> int:
        arr = []
        cur = head
        while cur:
            arr.append(cur.val)
            cur = cur.next

        i, j = 0, len(arr) - 1
        res = 0
        while i < j:
            res = max(res, arr[i] + arr[j])
            i, j = i + 1, j - 1

        return res

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def pairSum(self, head: Optional[ListNode]) -> int:
        slow, fast = head, head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next

        prev, cur = None, slow
        while cur:
            nxt = cur.next
            cur.next = prev
            prev = cur
            cur = nxt

        res = 0
        first, second = head, prev
        while second:
            res = max(res, first.val + second.val)
            first, second = first.next, second.next

        return res

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def pairSum(self, head: Optional[ListNode]) -> int:
        slow, fast = head, head
        prev = None

        while fast and fast.next:
            fast = fast.next.next
            tmp = slow.next
            slow.next = prev
            prev = slow
            slow = tmp

        res = 0
        while slow:
            res = max(res, prev.val + slow.val)
            prev = prev.next
            slow = slow.next

        return res