2130. Maximum Twin Sum of a Linked List

Author: remy727

README.md

@@ -186,6 +186,7 @@
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 | 1964 | Find the Longest Valid Obstacle Course at Each Position | [Ruby](./algorithms/ruby/1964-find-the-longest-valid-obstacle-course-at-each-position.rb) | Hard |
 | 2095 | Delete the Middle Node of a Linked List | [Ruby](./algorithms/ruby/2095-delete-the-middle-node-of-a-linked-list.rb) | Medium |
+| 2130 | Maximum Twin Sum of a Linked List | [Ruby](./algorithms/ruby/2130-maximum-twin-sum-of-a-linked-list.rb) | Medium |
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algorithms/ruby/2130-maximum-twin-sum-of-a-linked-list.rb

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+# frozen_string_literal: true
+
+# 2130. Maximum Twin Sum of a Linked List
+# https://leetcode.com/problems/maximum-twin-sum-of-a-linked-list
+# Medium
+
+=begin
+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.
+
+Constraints:
+The number of nodes in the list is an even integer in the range [2, 105].
+1 <= Node.val <= 105
+=end
+
+# Definition for singly-linked list.
+# class ListNode
+#     attr_accessor :val, :next
+#     def initialize(val = 0, _next = nil)
+#         @val = val
+#         @next = _next
+#     end
+# end
+# @param {ListNode} head
+# @return {Integer}
+def pair_sum(head)
+  slow = fast = head
+  stack = []
+
+  while fast&.next
+    stack << slow.val
+    slow = slow.next
+    fast = fast.next.next
+  end
+
+  max = 0
+  while (val = stack.pop)
+    max = [val + slow.val, max].max
+    slow = slow.next
+  end
+
+  max
+end