class Solution {
    public int pairSum(ListNode head) {
        int l=length(head), max=0, index;
        int[] arr = new int[l/2];
        for(int i=0; i<l; i++){
            index=i<l/2 ? i : l-i-1;
            arr[index] += head.val;
            head=head.next;
            max=Math.max(max, arr[index]);
        }
        return max;
    }
    public int length(ListNode node){
        int count=0;
        while(node!=null){
            count++;
            node=node.next;
        }
        return count;
    }
}