Linked Structures

Reverse a Singly Linked List

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

Algorithm

Basic Implementation

basic.rs 
struct ListNode {
	value: i32,
	next: Option<usize>,
}

fn add_node(nodes: &mut Vec<ListNode>, value: i32, next: Option<usize>) -> usize {
	let idx = nodes.len();
	nodes.push(ListNode { value, next });
	idx
}

fn main() {
	let mut nodes: Vec<ListNode> = Vec::new();
	let n5 = add_node(&mut nodes, 5, None);
	let n4 = add_node(&mut nodes, 4, Some(n5));
	let n3 = add_node(&mut nodes, 3, Some(n4));
	let n2 = add_node(&mut nodes, 2, Some(n3));
	let mut head: Option<usize> = Some(add_node(&mut nodes, 1, Some(n2)));

	let mut prev: Option<usize> = None;
	let mut cursor: Option<usize> = head;
	while let Some(c) = cursor {
		let next = nodes[c].next;
		nodes[c].next = prev;
		prev = Some(c);
		cursor = next;
	}
	head = prev;
	let mut cur = head;
	while let Some(idx) = cur {
		print!("{} -> ", nodes[idx].value);
		cur = nodes[idx].next;
	}
	println!("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

  • Rust: same three-pointer pattern as the other languages, but each pointer is an Option<usize> index into the Vec<ListNode> arena. The arena avoids the Box<ListNode> ownership-and-take dance the classic Rust 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 raw addresses.
three pointers `prev` starts `None`, `cursor` starts at `head`, `next` is the saved forward link.
rewire The rewire frame flips `cursor.next` from forward (toward `next`) to backward (toward `prev`).