LeetCode-in-Java
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 node0is the twin of node3, and node1is the twin of node2. These are the only nodes with twins forn = 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;
public class Solution {
public int pairSum(ListNode head) {
if (head == null) {
return 0;
}
int maxSum = Integer.MIN_VALUE;
ListNode slow = head;
ListNode fast = head;
while (fast != null && fast.next != null) {
slow = slow.next;
fast = fast.next.next;
}
if (slow.next == null) {
return head.val + slow.val;
}
ListNode tail = head;
ListNode pivot = reverse(slow);
while (pivot != null) {
maxSum = Math.max(maxSum, (tail.val + pivot.val));
tail = tail.next;
pivot = pivot.next;
}
return maxSum;
}
private ListNode reverse(ListNode head) {
ListNode tail = head;
ListNode prev = null;
while (tail != null) {
ListNode temp = tail.next;
tail.next = prev;
prev = tail;
tail = temp;
}
return prev;
}
}