maximum-twin-sum-of-a-linked-list

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Algorithms

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

<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>

The twin sum is defined as the sum of a node and its twin.

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

&nbsp;
Example 1:
<img alt="" src="https://assets.leetcode.com/uploads/2021/12/03/eg1drawio.png" style="width: 250px; height: 70px;" />
<pre>
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. 
</pre>

Example 2:
<img alt="" src="https://assets.leetcode.com/uploads/2021/12/03/eg2drawio.png" style="width: 250px; height: 70px;" />
<pre>
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. 
</pre>

Example 3:
<img alt="" src="https://assets.leetcode.com/uploads/2021/12/03/eg3drawio.png" style="width: 200px; height: 88px;" />
<pre>
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.
</pre>

&nbsp;
Constraints:

<ul>
	<li>The number of nodes in the list is an even integer in the range <code>[2, 105]</code>.</li>
	<li><code>1 &lt;= Node.val &lt;= 105</code></li>
</ul>

pairSum

Maximum Twin Sum of a Linked List

Reverse Linked List

reverse-linked-list

反转链表

Palindrome Linked List

palindrome-linked-list

回文链表

Middle of the Linked List

middle-of-the-linked-list

链表的中间结点

Stack

Linked List

Two Pointers

在一个大小为&nbsp;<code>n</code>&nbsp;且 <code>n</code>&nbsp;为&nbsp;偶数 的链表中，对于&nbsp;<code>0 &lt;= i &lt;= (n / 2) - 1</code>&nbsp;的 <code>i</code>&nbsp;，第&nbsp;<code>i</code>&nbsp;个节点（下标从 0&nbsp;开始）的孪生节点为第&nbsp;<code>(n-1-i)</code>&nbsp;个节点 。

<ul>
	<li>比方说，<code>n = 4</code>&nbsp;那么节点&nbsp;<code>0</code>&nbsp;是节点 <code>3</code>&nbsp;的孪生节点，节点 <code>1</code>&nbsp;是节点 <code>2</code>&nbsp;的孪生节点。这是长度为 <code>n = 4</code>&nbsp;的链表中所有的孪生节点。</li>
</ul>

孪生和&nbsp;定义为一个节点和它孪生节点两者值之和。

给你一个长度为偶数的链表的头节点&nbsp;<code>head</code>&nbsp;，请你返回链表的 最大孪生和&nbsp;。

&nbsp;

示例&nbsp;1：

<img alt="" src="https://assets.leetcode.com/uploads/2021/12/03/eg1drawio.png" style="width: 250px; height: 70px;">

<pre>输入：head = [5,4,2,1]
输出：6
解释：
节点 0 和节点 1 分别是节点 3 和 2 的孪生节点。孪生和都为 6 。
链表中没有其他孪生节点。
所以，链表的最大孪生和是 6 。
</pre>

示例 2：

<img alt="" src="https://assets.leetcode.com/uploads/2021/12/03/eg2drawio.png" style="width: 250px; height: 70px;">

<pre>输入：head = [4,2,2,3]
输出：7
解释：
链表中的孪生节点为：
- 节点 0 是节点 3 的孪生节点，孪生和为 4 + 3 = 7 。
- 节点 1 是节点 2 的孪生节点，孪生和为 2 + 2 = 4 。
所以，最大孪生和为 max(7, 4) = 7 。
</pre>

示例 3：

<img alt="" src="https://assets.leetcode.com/uploads/2021/12/03/eg3drawio.png" style="width: 200px; height: 88px;">

<pre>输入：head = [1,100000]
输出：100001
解释：
链表中只有一对孪生节点，孪生和为 1 + 100000 = 100001 。
</pre>

&nbsp;

提示：

<ul>
	<li>链表的节点数目是&nbsp;<code>[2, 105]</code>&nbsp;中的&nbsp;偶数&nbsp;。</li>
	<li><code>1 &lt;= Node.val &lt;= 105</code></li>
</ul>