LeetCode in Kotlin

2130. Maximum Twin Sum of a Linked List

Medium

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

Solution

import com_github_leetcode.ListNode

/*
 * Example:
 * var li = ListNode(5)
 * var v = li.`val`
 * Definition for singly-linked list.
 * class ListNode(var `val`: Int) {
 *     var next: ListNode? = null
 * }
 */
class Solution {
    fun pairSum(head: ListNode?): Int {
        if (head == null) {
            return 0
        }
        var maxSum = Int.MIN_VALUE
        var slow = head
        var fast = head
        while (fast != null && fast.next != null) {
            slow = slow!!.next
            fast = fast.next!!.next
        }
        if (slow!!.next == null) {
            return head.`val` + slow.`val`
        }
        var tail = head
        var pivot = reverse(slow)
        while (pivot != null) {
            maxSum = Math.max(maxSum, tail!!.`val` + pivot.`val`)
            tail = tail.next
            pivot = pivot.next
        }
        return maxSum
    }

    private fun reverse(head: ListNode?): ListNode? {
        var tail = head
        var prev: ListNode? = null
        while (tail != null) {
            val temp = tail.next
            tail.next = prev
            prev = tail
            tail = temp
        }
        return prev
    }
}

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