Maximum Twin Sum of a Linked List

给一个链表, 定义twin sum是前后对应的两个node的和, 求最大的twin sum.

/**
 * Definition for singly-linked list.
 * 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) {
        ListNode p = head;
        ListNode m = mid(head);
        ListNode r = rev(m);
        int res = 0;
        while(p != null){
            res = Math.max(res, p.val + r.val);
            p = p.next;
            r = r.next;
        }
        return res;
    }
    private ListNode rev(ListNode head) {
        ListNode newHead = null;
        while(head != null){
            ListNode next = head.next;
            head.next = newHead;
            newHead = head;
            head = next;
        }
        return newHead;
    }
    private ListNode mid(ListNode head) {
        ListNode dF = new ListNode(0);
        ListNode sF = new ListNode(0);
        dF.next = head;
        sF.next = head;
        while(dF.next != null && dF.next.next != null){
            sF = sF.next;
            dF = dF.next.next;
        }
        return sF;
    }
}