Maximum Twin Sum of a Linked List — Split and Reverse

# Python
def pairSum(head):
    # Find middle
    slow = fast = head
    while fast and fast.next:
        slow = slow.next; fast = fast.next.next
    # Reverse second half
    prev, cur = None, slow
    while cur:
        nxt = cur.next; cur.next = prev; prev = cur; cur = nxt
    # Compute max twin sum
    ans = 0
    first, second = head, prev
    while second:
        ans = max(ans, first.val + second.val)
        first = first.next; second = second.next
    return ans

// Java
public int pairSum(ListNode head) {
    ListNode slow = head, fast = head;
    while (fast != null && fast.next != null) { slow = slow.next; fast = fast.next.next; }
    ListNode prev = null, cur = slow;
    while (cur != null) { ListNode nxt = cur.next; cur.next = prev; prev = cur; cur = nxt; }
    int ans = 0;
    ListNode l1 = head, l2 = prev;
    while (l2 != null) { ans = Math.max(ans, l1.val + l2.val); l1 = l1.next; l2 = l2.next; }
    return ans;
}