Stacks and Queues

Queue from Two Stacks

Implement queue behavior with an input stack and an output stack.

Algorithm

The replay uses the same three values in every language, so this SQL DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.sql 
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CREATE TABLE in_stack(pos INTEGER PRIMARY KEY, val INTEGER);
INSERT INTO in_stack(pos, val) VALUES (0, 10), (1, 20), (2, 30);
CREATE TABLE out_stack(pos INTEGER PRIMARY KEY, val INTEGER);
INSERT INTO out_stack(pos, val)
SELECT 2 - pos, val FROM in_stack ORDER BY pos DESC;
SELECT GROUP_CONCAT(val, ' -> ')
FROM (SELECT val FROM out_stack ORDER BY pos DESC);

Complexity

  • Time: O(1) amortized per operation
  • Space: O(n)

Implementation notes

  • Keep the explicit stack/queue operations. Library shortcuts that only produce the final list hide the data-structure behavior this lesson is meant to replay.
  • The final output uses a deterministic a -> b -> c format for cross-language comparison.
input stack Enqueue pushes new values onto the input stack.
output stack When the output stack is empty, transferring all input values reverses them into dequeue order.