maximum-twin-sum-of-a-linked-list_2130.py

# In a linked list of size n, where n is even, the ith node (0-indexed) of the linked list is known as the twin of the (n-1-i)th node, if 0 <= i <= (n / 2) - 1.

# For example, if n = 4, then node 0 is the twin of node 3, and node 1 is the twin of node 2. These are the only nodes with twins for n = 4.
# The twin sum is defined as the sum of a node and its twin.

# Given the head of a linked list with even length, return the maximum twin sum of the linked list.


# Example 1:

# 
# Input: head = [5,4,2,1]
# Output: 6
# Explanation:
# Nodes 0 and 1 are the twins of nodes 3 and 2, respectively. All have twin sum = 6.
# There are no other nodes with twins in the linked list.
# Thus, the maximum twin sum of the linked list is 6.
# Example 2:

# 
# Input: head = [4,2,2,3]
# Output: 7
# Explanation:
# The nodes with twins present in this linked list are:
# - Node 0 is the twin of node 3 having a twin sum of 4 + 3 = 7.
# - Node 1 is the twin of node 2 having a twin sum of 2 + 2 = 4.
# Thus, the maximum twin sum of the linked list is max(7, 4) = 7.
# Example 3:

# 
# Input: head = [1,100000]
# Output: 100001
# Explanation:
# There is only one node with a twin in the linked list having twin sum of 1 + 100000 = 100001.

# ---------------------------------------Runtime 715 ms Beats 63.77% Memory 56.8 MB Beats 48.82%---------------------------------------


from typing import Optional


class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next


class Solution:
    def pairSum(self, head: Optional[ListNode]) -> int:
        node = head
        nums = []
        while node:
            nums.append(node.val)
            node = node.next

        res = 0
        start, end = 0, len(nums) - 1
        while start < end:
            res = max(res, nums[start] + nums[end])
            start += 1
            end -= 1
        return res