LeetCode #3570 — EASY

Find Books with No Available Copies

Build confidence with an intuition-first walkthrough focused on core interview patterns fundamentals.

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The Problem

Problem Statement

Table: library_books

+------------------+---------+
| Column Name      | Type    |
+------------------+---------+
| book_id          | int     |
| title            | varchar |
| author           | varchar |
| genre            | varchar |
| publication_year | int     |
| total_copies     | int     |
+------------------+---------+
book_id is the unique identifier for this table.
Each row contains information about a book in the library, including the total number of copies owned by the library.

Table: borrowing_records

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| record_id     | int     |
| book_id       | int     |
| borrower_name | varchar |
| borrow_date   | date    |
| return_date   | date    |
+---------------+---------+
record_id is the unique identifier for this table.
Each row represents a borrowing transaction and return_date is NULL if the book is currently borrowed and hasn't been returned yet.

Write a solution to find all books that are currently borrowed (not returned) and have zero copies available in the library.

  • A book is considered currently borrowed if there exists a borrowing record with a NULL return_date

Return the result table ordered by current borrowers in descending order, then by book title in ascending order.

The result format is in the following example.

Example:

Input:

library_books table:

+---------+------------------------+------------------+----------+------------------+--------------+
| book_id | title                  | author           | genre    | publication_year | total_copies |
+---------+------------------------+------------------+----------+------------------+--------------+
| 1       | The Great Gatsby       | F. Scott         | Fiction  | 1925             | 3            |
| 2       | To Kill a Mockingbird  | Harper Lee       | Fiction  | 1960             | 3            |
| 3       | 1984                   | George Orwell    | Dystopian| 1949             | 1            |
| 4       | Pride and Prejudice    | Jane Austen      | Romance  | 1813             | 2            |
| 5       | The Catcher in the Rye | J.D. Salinger    | Fiction  | 1951             | 1            |
| 6       | Brave New World        | Aldous Huxley    | Dystopian| 1932             | 4            |
+---------+------------------------+------------------+----------+------------------+--------------+

borrowing_records table:

+-----------+---------+---------------+-------------+-------------+
| record_id | book_id | borrower_name | borrow_date | return_date |
+-----------+---------+---------------+-------------+-------------+
| 1         | 1       | Alice Smith   | 2024-01-15  | NULL        |
| 2         | 1       | Bob Johnson   | 2024-01-20  | NULL        |
| 3         | 2       | Carol White   | 2024-01-10  | 2024-01-25  |
| 4         | 3       | David Brown   | 2024-02-01  | NULL        |
| 5         | 4       | Emma Wilson   | 2024-01-05  | NULL        |
| 6         | 5       | Frank Davis   | 2024-01-18  | 2024-02-10  |
| 7         | 1       | Grace Miller  | 2024-02-05  | NULL        |
| 8         | 6       | Henry Taylor  | 2024-01-12  | NULL        |
| 9         | 2       | Ivan Clark    | 2024-02-12  | NULL        |
| 10        | 2       | Jane Adams    | 2024-02-15  | NULL        |
+-----------+---------+---------------+-------------+-------------+

Output:

+---------+------------------+---------------+-----------+------------------+-------------------+
| book_id | title            | author        | genre     | publication_year | current_borrowers |
+---------+------------------+---------------+-----------+------------------+-------------------+
| 1       | The Great Gatsby | F. Scott      | Fiction   | 1925             | 3                 | 
| 3       | 1984             | George Orwell | Dystopian | 1949             | 1                 |
+---------+------------------+---------------+-----------+------------------+-------------------+

Explanation:

  • The Great Gatsby (book_id = 1):
    • Total copies: 3
    • Currently borrowed by Alice Smith, Bob Johnson, and Grace Miller (3 borrowers)
    • Available copies: 3 - 3 = 0
    • Included because available_copies = 0
  • 1984 (book_id = 3):
    • Total copies: 1
    • Currently borrowed by David Brown (1 borrower)
    • Available copies: 1 - 1 = 0
    • Included because available_copies = 0
  • Books not included:
    • To Kill a Mockingbird (book_id = 2): Total copies = 3, current borrowers = 2, available = 1
    • Pride and Prejudice (book_id = 4): Total copies = 2, current borrowers = 1, available = 1
    • The Catcher in the Rye (book_id = 5): Total copies = 1, current borrowers = 0, available = 1
    • Brave New World (book_id = 6): Total copies = 4, current borrowers = 1, available = 3
  • Result ordering:
    • The Great Gatsby appears first with 3 current borrowers
    • 1984 appears second with 1 current borrower

Output table is ordered by current_borrowers in descending order, then by book_title in ascending order.

Roadmap

  1. Brute Force Baseline
  2. Core Insight
  3. Algorithm Walkthrough
  4. Edge Cases
  5. Full Annotated Code
  6. Interactive Study Demo
  7. Complexity Analysis
Step 01

Brute Force Baseline

Problem summary: Table: library_books +------------------+---------+ | Column Name | Type | +------------------+---------+ | book_id | int | | title | varchar | | author | varchar | | genre | varchar | | publication_year | int | | total_copies | int | +------------------+---------+ book_id is the unique identifier for this table. Each row contains information about a book in the library, including the total number of copies owned by the library. Table: borrowing_records +---------------+---------+ | Column Name | Type | +---------------+---------+ | record_id | int | | book_id | int | | borrower_name | varchar | | borrow_date | date | | return_date | date | +---------------+---------+ record_id is the unique identifier for this table. Each row represents a borrowing transaction and return_date is NULL if the book is currently borrowed and hasn't been returned yet. Write a solution to find all books that

Baseline thinking

Start with the most direct exhaustive search. That gives a correctness anchor before optimizing.

Pattern signal: General problem-solving

Example 1

{"headers":{"library_books":["book_id","title","author","genre","publication_year","total_copies"],"borrowing_records":["record_id","book_id","borrower_name","borrow_date","return_date"]},"rows":{"library_books":[[1,"The Great Gatsby","F. Scott","Fiction",1925,3],[2,"To Kill a Mockingbird","Harper Lee","Fiction",1960,3],[3,"1984","George Orwell","Dystopian",1949,1],[4,"Pride and Prejudice","Jane Austen","Romance",1813,2],[5,"The Catcher in the Rye","J.D. Salinger","Fiction",1951,1],[6,"Brave New World","Aldous Huxley","Dystopian",1932,4]],"borrowing_records":[[1,1,"Alice Smith","2024-01-15",null],[2,1,"Bob Johnson","2024-01-20",null],[3,2,"Carol White","2024-01-10","2024-01-25"],[4,3,"David Brown","2024-02-01",null],[5,4,"Emma Wilson","2024-01-05",null],[6,5,"Frank Davis","2024-01-18","2024-02-10"],[7,1,"Grace Miller","2024-02-05",null],[8,6,"Henry Taylor","2024-01-12",null],[9,2,"Ivan Clark","2024-02-12",null],[10,2,"Jane Adams","2024-02-15",null]]}}
Step 02

Core Insight

What unlocks the optimal approach

  • No official hints in dataset. Start from constraints and look for a monotonic or reusable state.
Interview move: turn each hint into an invariant you can check after every iteration/recursion step.
Step 03

Algorithm Walkthrough

Iteration Checklist

  1. Define state (indices, window, stack, map, DP cell, or recursion frame).
  2. Apply one transition step and update the invariant.
  3. Record answer candidate when condition is met.
  4. Continue until all input is consumed.
Use the first example testcase as your mental trace to verify each transition.
Step 04

Edge Cases

Minimum Input
Single element / shortest valid input
Validate boundary behavior before entering the main loop or recursion.
Duplicates & Repeats
Repeated values / repeated states
Decide whether duplicates should be merged, skipped, or counted explicitly.
Extreme Constraints
Upper-end input sizes
Re-check complexity target against constraints to avoid time-limit issues.
Invalid / Corner Shape
Empty collections, zeros, or disconnected structures
Handle special-case structure before the core algorithm path.
Step 05

Full Annotated Code

Source-backed implementations are provided below for direct study and interview prep.

// Accepted solution for LeetCode #3570: Find Books with No Available Copies
// Auto-generated Java example from py.
class Solution {
    public void exampleSolution() {
    }
}
// Reference (py):
// # Accepted solution for LeetCode #3570: Find Books with No Available Copies
// import pandas as pd
// 
// 
// def find_books_with_no_available_copies(
//     library_books: pd.DataFrame, borrowing_records: pd.DataFrame
// ) -> pd.DataFrame:
//     current_borrowers = (
//         borrowing_records[borrowing_records["return_date"].isna()]
//         .groupby("book_id")
//         .size()
//         .rename("current_borrowers")
//         .reset_index()
//     )
// 
//     merged = library_books.merge(current_borrowers, on="book_id", how="inner")
//     fully_borrowed = merged[merged["current_borrowers"] == merged["total_copies"]]
//     fully_borrowed = fully_borrowed.sort_values(
//         by=["current_borrowers", "title"], ascending=[False, True]
//     )
// 
//     cols = [
//         "book_id",
//         "title",
//         "author",
//         "genre",
//         "publication_year",
//         "current_borrowers",
//     ]
//     return fully_borrowed[cols].reset_index(drop=True)
Step 06

Interactive Study Demo

Use this to step through a reusable interview workflow for this problem.

Press Step or Run All to begin.
Step 07

Complexity Analysis

Time
O(n)
Space
O(1)

Approach Breakdown

BRUTE FORCE
O(n²) time
O(1) space

Two nested loops check every pair or subarray. The outer loop fixes a starting point, the inner loop extends or searches. For n elements this gives up to n²/2 operations. No extra space, but the quadratic time is prohibitive for large inputs.

OPTIMIZED
O(n) time
O(1) space

Most array problems have an O(n²) brute force (nested loops) and an O(n) optimal (single pass with clever state tracking). The key is identifying what information to maintain as you scan: a running max, a prefix sum, a hash map of seen values, or two pointers.

Shortcut: If you are using nested loops on an array, there is almost always an O(n) solution. Look for the right auxiliary state.
Coach Notes

Common Mistakes

Review these before coding to avoid predictable interview regressions.

Off-by-one on range boundaries

Wrong move: Loop endpoints miss first/last candidate.

Usually fails on: Fails on minimal arrays and exact-boundary answers.

Fix: Re-derive loops from inclusive/exclusive ranges before coding.