Ruby

1# Definition for singly-linked list.
2# class ListNode
3#     attr_accessor :val, :next
4#     def initialize(val = 0, _next = nil)
5#         @val = val
6#         @next = _next
7#     end
8# end
9# @param {ListNode} head
10# @param {Integer} n
11# @return {ListNode}
12def remove_nth_from_end(head, n)
13    cnt = 0
14    curr = head
15    
16    while curr
17        cnt += 1
18        curr = curr.next
19    end 
20    
21    if cnt == n
22        head = head.next    
23    else
24        cnt -= n
25
26        curr = head
27        (cnt-1).times {curr = curr.next}
28        curr.next = curr.next.next
29    end
30    
31    head
32end