Linked Structures

Reverse a Singly Linked List

Walk the list with three pointers: prev, cursor, and next_idx. Each iteration saves cursor.next_idx, re-points cursor.next_idx backward to prev, then advances prev = cursor; cursor = next_idx.

Algorithm

Basic Implementation

basic.rb 
ListNode = Struct.new(:value, :next_idx)

def add_node(nodes, value, next_idx)
	idx = nodes.length
	nodes << ListNode.new(value, next_idx)
	idx
end

nodes = []
n5 = add_node(nodes, 5, -1)
n4 = add_node(nodes, 4, n5)
n3 = add_node(nodes, 3, n4)
n2 = add_node(nodes, 2, n3)
head = add_node(nodes, 1, n2)

prev = -1
cursor = head
while cursor != -1
	next_idx = nodes[cursor].next_idx
	nodes[cursor].next_idx = prev
	prev = cursor
	cursor = next_idx
end
head = prev
cur = head
while cur != -1
	print "#{nodes[cur].value} -> "
	cur = nodes[cur].next_idx
end
puts "nil"

The three-pointer loop saves the forward link, flips one next pointer, then advances prev and cursor.

Step 1 - Save the first forward link

prev starts at null, cursor is node(1), and nxt saves node(2).

Initial 1 -> 2 -> 3 -> 4 -> 5 chain with prev, cursor, and nxt named.prevcursornxtnullnode(1)node(2)node(3)node(4)node(5)

Step 2 - Flip node(1)

Set node(1).next to prev, making the reversed prefix 1 -> null.

After the first flip, prev points at node(1) and cursor advances to node(2).prevcursornode(1)nullnode(2)node(3)node(4)node(5)

Step 3 - Reversed prefix reaches 3

After three flips, the prefix is 3 -> 2 -> 1 -> null and cursor is node(4).

Middle of the reverse: prefix 3 -> 2 -> 1, suffix 4 -> 5.prevcursornode(3)node(2)node(1)nullnode(4)node(5)

Step 4 - Done

When cursor reaches null, prev is the new head: 5 -> 4 -> 3 -> 2 -> 1 -> null.

Final reversed list.headnode(5)node(4)node(3)node(2)node(1)null

Complexity

  • Time: O(n)
  • Space: O(1)

Implementation notes

  • Ruby: same three-pointer pattern as the other languages, but each pointer is an Integer index into the Array arena of ListNode structs (-1 is the sentinel). The arena avoids the recursive class-reference dance the classic idiom requires while keeping the algorithm visible.
  • head = prev at the end re-points the head at the old tail; the arena keeps every node alive throughout the reversal.
  • The replay shows all three pointers each frame and a distinct rewire frame between save and advance, with node(<value>) labels instead of runtime references.
three pointers `prev` starts `-1`, `cursor` starts at `head`, `next_idx` is the saved forward link.
rewire The rewire frame flips `cursor.next_idx` from forward (toward `next_idx`) to backward (toward `prev`).