This solution done by the list

1. get all the element in the list
2. get the variable 0 and the length of the list
3. sum both the variable
4. increase the first one and decrease the last

def pairSum(head):
    vec_list = []
    while head:
        vec_list.append(head.value)
        head = head.next
    
    max_sum = 0
    len_vec_list = len(vec_list) -1
    i = 0 
    j = len_vec_list
    while i<j:
      
        cur_sum = vec_list[i] + vec_list[j]
        max_sum = max(max_sum,cur_sum )
        i = i+1
        j = j-1
    return max_sum