2130. Maximum Twin Sum of a Linked List

Medium
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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:
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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:
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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:
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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

解題

把數值都存進陣列後暴力解。
/**
* Definition for singly-linked list.
* type ListNode struct {
* Val int
* Next *ListNode
* }
*/
func pairSum(head *ListNode) int {
arr := make([]int, 0)
​
for head != nil {
arr = append(arr, head.Val)
head = head.Next
}
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ans := 0
for i:=0; i < len(arr) / 2 ; i++ {
if arr[i] + arr[len(arr) - 1 - i] > ans {
ans = arr[i] + arr[len(arr) - 1 - i]
}
}
​
return ans
}
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