2130. Maximum Twin Sum of a Linked List

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# 題目敘述

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.

# 解題思路

# Solution

// Definition for singly-linked list.

import java.util.Deque;

import java.util.LinkedList;

public class ListNode {

    int val;

    ListNode next;

    ListNode() {}

    ListNode(int val) { this.val = val; }

    ListNode(int val, ListNode next) { this.val = val; this.next = next; }

}

class Solution {

    public int pairSum(ListNode head) {

        Deque<Integer> dq = new LinkedList<>();

        ListNode curr = new ListNode();

        curr = head;

        while(curr != null){

            dq.add(curr.val);

            curr = curr.next;

        }

        int max = Integer.MIN_VALUE;

        while(!dq.isEmpty() && dq.size() >= 2){

            max = Math.max(max, dq.pollFirst() + dq.pollLast());

        }

        return max;

    }

}