Fix the Pagination Helper

paginate(items, page, pageSize) returns the items for a 1-based page number: page 1 is the first pageSize items. Pages past the end return an empty list.

Users report that page 1 skips the first results and the last page is always empty. Find the bug and fix it.

Example 1

Input: items = [1, 2, 3, 4, 5], page = 1, pageSize = 2
Output: [1, 2]
Explanation: The buggy version returns [3, 4].

Example 2

Input: items = [1, 2, 3, 4, 5], page = 3, pageSize = 2
Output: [5]

Constraints

page >= 1, pageSize >= 1

The buggy code

function paginate(items, page, pageSize) {
  const start = page * pageSize;
  return items.slice(start, start + pageSize);
}

function paginate(items: number[], page: number, pageSize: number): number[] {
  const start = page * pageSize;
  return items.slice(start, start + pageSize);
}

def paginate(items: list[int], page: int, page_size: int) -> list[int]:
    start = page * page_size
    return items[start:start + page_size]

<?php

function paginate(array $items, int $page, int $pageSize): array
{
    $start = $page * $pageSize;
    return array_slice($items, $start, $pageSize);
}

class Solution {
    public int[] paginate(int[] items, int page, int pageSize) {
        int start = Math.min(page * pageSize, items.length);
        int end = Math.min(start + pageSize, items.length);
        return Arrays.copyOfRange(items, start, end);
    }
}

Solution and explanation

Show the bug and the fix

Time O(pageSize) · Space O(pageSize)

The bug: page numbers are 1-based but the offset is computed as page x pageSize, which is the 0-based formula. Every page is shifted by one: page 1 shows page 2, and the real last page is unreachable.

The fix: offset = (page - 1) x pageSize. This is the same arithmetic as SQL LIMIT pageSize OFFSET (page - 1) * pageSize; for large tables, mention that keyset (cursor) pagination avoids scanning skipped rows.

Fixed version

function paginate(items, page, pageSize) {
  const start = (page - 1) * pageSize; // pages are 1-based
  return items.slice(start, start + pageSize);
}

function paginate(items: number[], page: number, pageSize: number): number[] {
  const start = (page - 1) * pageSize; // pages are 1-based
  return items.slice(start, start + pageSize);
}

def paginate(items: list[int], page: int, page_size: int) -> list[int]:
    start = (page - 1) * page_size  # pages are 1-based
    return items[start:start + page_size]

<?php

function paginate(array $items, int $page, int $pageSize): array
{
    $start = ($page - 1) * $pageSize; // pages are 1-based
    return array_slice($items, $start, $pageSize);
}

class Solution {
    public int[] paginate(int[] items, int page, int pageSize) {
        int start = Math.min((page - 1) * pageSize, items.length); // pages are 1-based
        int end = Math.min(start + pageSize, items.length);
        return Arrays.copyOfRange(items, start, end);
    }
}

Common mistakes

Computing the end index without clamping it to the list length (some languages throw).

Follow-up questions

Return totalPages too (ceil(n / pageSize)).
Why does OFFSET 100000 get slow in SQL, and what is keyset pagination?

Example 1

Example 2

Constraints

The buggy code

Hints

Find and fix the bug

Solution and explanation

Fixed version

Common mistakes

Follow-up questions

Example 1

Example 2

Constraints

The buggy code

Hints

Find and fix the bug

Solution and explanation

Fixed version

Common mistakes

Follow-up questions

More Code Clinic: Find the Bug problems