Linked Structures

Delete by Value

Find the node before the target value and rewire its next pointer so the target node leaves the chain.

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.

predecessor Deletion needs the node before the one being removed.
rewiring The predecessor skips the target and points at the target's next node.

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, 4), (4, 40, NULL);
UPDATE node SET next_idx = (SELECT next_idx FROM node WHERE val = 30) WHERE idx = 2;
DELETE FROM node WHERE val = 30;
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 -> 40 -> null, target ← 30

    1.mode list2.headers off
    values this step10 -> 20 -> 30 -> 40 -> nullhead30target

  2. cursor ← node(20)

    5UPDATE node SET next_idx = (SELECT next_idx FROM node WHERE val = 30) WHERE idx = 2;6DELETE FROM node WHERE val = 30;7WITH RECURSIVE walk(idx, line) AS (
    values this stepnode(10) → node(20)cursor

  3. cursor.next ← node(40)

    2.headers off3CREATE TABLE node(idx INTEGER PRIMARY KEY, val INTEGER, next_idx INTEGER);4INSERT INTO node(idx, val, next_idx) VALUES (1, 10, 2), (2, 20, 3), (3, 30, 4), (4, 40, NULL);
    values this stepnode(30) → node(40)cursor.next

  4. stdout ← 10 -> 20 -> 40 -> null

    4INSERT INTO node(idx, val, next_idx) VALUES (1, 10, 2), (2, 20, 3), (3, 30, 4), (4, 40, NULL);5UPDATE node SET next_idx = (SELECT next_idx FROM node WHERE val = 30) WHERE idx = 2;6DELETE FROM node WHERE val = 30;
    values this step10 -> 20 -> 40 -> nullstdout10 -> 20 -> 40 -> nullhead

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.