Linked Structures

Traverse and Print

Walk from the head pointer to null, visiting each node exactly once without random indexing.

Algorithm

The replay labels nodes by value, such as node(20), and never exposes object identity or memory addresses. This SQL DSA implementation uses the same small chain as the rest of the DSA track.

cursor A cursor reference names the node currently being visited.
null stop Traversal ends when the cursor reaches the null marker.

Basic Implementation

basic.sql
Replay: real traced execution (multi-file project)
.mode list
.headers off
CREATE TABLE node(idx INTEGER PRIMARY KEY, val INTEGER, next_idx INTEGER);
INSERT INTO node(idx, val, next_idx) VALUES (1, 10, 2), (2, 20, 3), (3, 30, NULL);
WITH RECURSIVE walk(idx, line) AS (
  SELECT 1, CAST(val AS TEXT) || ' -> ' FROM node WHERE idx = 1
  UNION ALL
  SELECT node.idx, walk.line || CAST(node.val AS TEXT) || ' -> '
  FROM walk JOIN node ON node.idx = (SELECT next_idx FROM node WHERE idx = walk.idx)
)
SELECT line || 'null' FROM walk
ORDER BY LENGTH(line) DESC LIMIT 1;

  1. head ← 10 -> 20 -> 30 -> null

    1.mode list2.headers off
    values this step10 -> 20 -> 30 -> nullhead

  2. output ← 10

    4INSERT INTO node(idx, val, next_idx) VALUES (1, 10, 2), (2, 20, 3), (3, 30, NULL);5WITH RECURSIVE walk(idx, line) AS (6  SELECT 1, CAST(val AS TEXT) || ' -> ' FROM node WHERE idx = 1
    values this step10outputnode(10)cursor

  3. output ← 10 -> 20

    4INSERT INTO node(idx, val, next_idx) VALUES (1, 10, 2), (2, 20, 3), (3, 30, NULL);5WITH RECURSIVE walk(idx, line) AS (6  SELECT 1, CAST(val AS TEXT) || ' -> ' FROM node WHERE idx = 1
    values this step10 -> 20outputnode(20)cursor

  4. output ← 10 -> 20 -> 30 -> null

    4INSERT INTO node(idx, val, next_idx) VALUES (1, 10, 2), (2, 20, 3), (3, 30, NULL);5WITH RECURSIVE walk(idx, line) AS (6  SELECT 1, CAST(val AS TEXT) || ' -> ' FROM node WHERE idx = 1
    values this step10 -> 20 -> 30 -> nulloutputnode(30)cursor

  5. stdout ← 10 -> 20 -> 30 -> null

    5WITH RECURSIVE walk(idx, line) AS (6  SELECT 1, CAST(val AS TEXT) || ' -> ' FROM node WHERE idx = 17  UNION ALL
    values this step10 -> 20 -> 30 -> nullstdout10 -> 20 -> 30 -> nulloutput

Complexity

  • Time: O(n)
  • Space: O(1)

Implementation notes

  • Keep the explicit node and pointer/reference operations; array shortcuts hide the linked-list state this lesson is meant to replay.
  • The final output prints the chain in a deterministic a -> b -> null form for cross-language comparison.