Maximum Twin Sum of a Linked List
<p>In a linked list of size <code>n</code>, where <code>n</code> is <strong>even</strong>, the <code>i<sup>th</sup></code> node (<strong>0-indexed</strong>) of the linked list is known as the <strong>twin</strong> of the <code>(n-1-i)<sup>th</sup></code> node, if <code>0 &lt;= i &lt;= (n / 2) - 1</code>.</p>

<ul>
	<li>For example, if <code>n = 4</code>, then node <code>0</code> is the twin of node <code>3</code>, and node <code>1</code> is the twin of node <code>2</code>. These are the only nodes with twins for <code>n = 4</code>.</li>
</ul>

<p>The <strong>twin sum </strong>is defined as the sum of a node and its twin.</p>

<p>Given the <code>head</code> of a linked list with even length, return <em>the <strong>maximum twin sum</strong> of the linked list</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/2100-2199/2130.Maximum%20Twin%20Sum%20of%20a%20Linked%20List/images/eg1drawio.png" style="width: 250px; height: 70px;" />
<pre>
<strong>Input:</strong> head = [5,4,2,1]
<strong>Output:</strong> 6
<strong>Explanation:</strong>
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. 
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/2100-2199/2130.Maximum%20Twin%20Sum%20of%20a%20Linked%20List/images/eg2drawio.png" style="width: 250px; height: 70px;" />
<pre>
<strong>Input:</strong> head = [4,2,2,3]
<strong>Output:</strong> 7
<strong>Explanation:</strong>
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. 
</pre>

<p><strong class="example">Example 3:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/2100-2199/2130.Maximum%20Twin%20Sum%20of%20a%20Linked%20List/images/eg3drawio.png" style="width: 200px; height: 88px;" />
<pre>
<strong>Input:</strong> head = [1,100000]
<strong>Output:</strong> 100001
<strong>Explanation:</strong>
There is only one node with a twin in the linked list having twin sum of 1 + 100000 = 100001.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the list is an <strong>even</strong> integer in the range <code>[2, 10<sup>5</sup>]</code>.</li>
	<li><code>1 &lt;= Node.val &lt;= 10<sup>5</sup></code></li>
</ul>
Stack; Linked List; Two Pointers
// Definition for singly-linked list.
// #[derive(PartialEq, Eq, Clone, Debug)]
// pub struct ListNode {
//   pub val: i32,
//   pub next: Option<Box<ListNode>>
// }
//
// impl ListNode {
//   #[inline]
//   fn new(val: i32) -> Self {
//     ListNode {
//       next: None,
//       val
//     }
//   }
// }
impl Solution {
    pub fn pair_sum(head: Option<Box<ListNode>>) -> i32 {
        let mut arr = Vec::new();
        let mut node = &head;
        while node.is_some() {
            let t = node.as_ref().unwrap();
            arr.push(t.val);
            node = &t.next;
        }
        let n = arr.len();
        let mut ans = 0;
        for i in 0..n >> 1 {
            ans = ans.max(arr[i] + arr[n - 1 - i]);
        }
        ans
    }
}
