feat: solve No.2130

Author: BaffinLee

Diff for: 2101-2200/2130. Maximum Twin Sum of a Linked List.md

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+# 2130. Maximum Twin Sum of a Linked List
+
+- Difficulty: Medium.
+- Related Topics: Linked List, Two Pointers, Stack.
+- Similar Questions: Reverse Linked List, Palindrome Linked List, Middle of the Linked List.
+
+## Problem
+
+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:
+
+![](https://assets.leetcode.com/uploads/2021/12/03/eg1drawio.png)
+
+```
+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:
+
+![](https://assets.leetcode.com/uploads/2021/12/03/eg2drawio.png)
+
+```
+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:
+
+![](https://assets.leetcode.com/uploads/2021/12/03/eg3drawio.png)
+
+```
+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.
+```
+
+ 
+**Constraints:**
+
+
+	
+- The number of nodes in the list is an **even** integer in the range `[2, 105]`.
+	
+- `1 <= Node.val <= 105`
+
+
+
+## Solution
+
+```javascript
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val, next) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.next = (next===undefined ? null : next)
+ * }
+ */
+/**
+ * @param {ListNode} head
+ * @return {number}
+ */
+var pairSum = function(head) {
+    var num = 0;
+    var node = head;
+    while (node) {
+        num++;
+        node = node.next;
+    }
+
+    var stack = [];
+    var sum = 0;
+    var i = 0;
+    while (i < num) {
+        if (i < num / 2) {
+            stack.push(head.val);
+        } else {
+            sum = Math.max(sum, head.val + stack.pop());
+        }
+        i += 1;
+        head = head.next;
+    }
+
+    return sum;
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(n).