2130. Maximum Twin Sum of a Linked List

class Solution {
    public int pairSum(ListNode head) {
        // Find middle
        ListNode slow = head;
        ListNode fast = head;

        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        // Reverse second half
        ListNode prev = null;
        while (slow != null) {
            ListNode next = slow.next;
            slow.next = prev;
            prev = slow;
            slow = next;
        }
        // Calculate maximum twin sum
        int ans = 0;
        ListNode first = head;
        ListNode second = prev;

        while (second != null) {
            ans = Math.max(ans, first.val + second.val);
            first = first.next;
            second = second.next;
        }
        return ans;
    }
}

class Solution:
    def pairSum(self, head):
        slow = head
        fast = head

        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next

        prev = None
        while slow:
            nxt = slow.next
            slow.next = prev
            prev = slow
            slow = nxt

        ans = 0
        first = head
        second = prev

        while second:
            ans = max(ans, first.val + second.val)
            first = first.next
            second = second.next

        return ans

class Solution {
public:
    int pairSum(ListNode* head) {

        ListNode* slow = head;
        ListNode* fast = head;

        while (fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
        }

        ListNode* prev = nullptr;

        while (slow) {
            ListNode* nxt = slow->next;
            slow->next = prev;
            prev = slow;
            slow = nxt;
        }

        int ans = 0;

        ListNode* first = head;
        ListNode* second = prev;

        while (second) {
            ans = max(ans, first->val + second->val);
            first = first->next;
            second = second->next;
        }

        return ans;
    }
};

var pairSum = function(head) {

    let slow = head;
    let fast = head;

    while (fast && fast.next) {
        slow = slow.next;
        fast = fast.next.next;
    }

    let prev = null;

    while (slow) {
        let next = slow.next;
        slow.next = prev;
        prev = slow;
        slow = next;
    }

    let ans = 0;
    let first = head;
    let second = prev;

    while (second) {
        ans = Math.max(ans, first.val + second.val);
        first = first.next;
        second = second.next;
    }

    return ans;
};