LeetCode #3465 — EASY

Find Products with Valid Serial Numbers

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

The Problem

Problem Statement

Table: products

+--------------+------------+
| Column Name  | Type       |
+--------------+------------+
| product_id   | int        |
| product_name | varchar    |
| description  | varchar    |
+--------------+------------+
(product_id) is the unique key for this table.
Each row in the table represents a product with its unique ID, name, and description.

Write a solution to find all products whose description contains a valid serial number pattern. A valid serial number follows these rules:

It starts with the letters SN (case-sensitive).
Followed by exactly 4 digits.
It must have a hyphen (-) followed by exactly 4 digits.
The serial number must be within the description (it may not necessarily start at the beginning).

Return the result table ordered by product_id in ascending order.

The result format is in the following example.

Example:

Input:

products table:

+------------+--------------+------------------------------------------------------+
| product_id | product_name | description                                          |
+------------+--------------+------------------------------------------------------+
| 1          | Widget A     | This is a sample product with SN1234-5678            |
| 2          | Widget B     | A product with serial SN9876-1234 in the description |
| 3          | Widget C     | Product SN1234-56789 is available now                |
| 4          | Widget D     | No serial number here                                |
| 5          | Widget E     | Check out SN4321-8765 in this description            |
+------------+--------------+------------------------------------------------------+

Output:

+------------+--------------+------------------------------------------------------+
| product_id | product_name | description                                          |
+------------+--------------+------------------------------------------------------+
| 1          | Widget A     | This is a sample product with SN1234-5678            |
| 2          | Widget B     | A product with serial SN9876-1234 in the description |
| 5          | Widget E     | Check out SN4321-8765 in this description            |
+------------+--------------+------------------------------------------------------+

Explanation:

Product 1: Valid serial number SN1234-5678
Product 2: Valid serial number SN9876-1234
Product 3: Invalid serial number SN1234-56789 (contains 5 digits after the hyphen)
Product 4: No serial number in the description
Product 5: Valid serial number SN4321-8765

The result table is ordered by product_id in ascending order.

Step 01

Brute Force Baseline

Problem summary: Table: products +--------------+------------+ | Column Name | Type | +--------------+------------+ | product_id | int | | product_name | varchar | | description | varchar | +--------------+------------+ (product_id) is the unique key for this table. Each row in the table represents a product with its unique ID, name, and description. Write a solution to find all products whose description contains a valid serial number pattern. A valid serial number follows these rules: It starts with the letters SN (case-sensitive). Followed by exactly 4 digits. It must have a hyphen (-) followed by exactly 4 digits. The serial number must be within the description (it may not necessarily start at the beginning). Return the result table ordered by product_id in ascending order. The result format is in the following example.

Baseline thinking

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

Pattern signal: General problem-solving

Example 1

{"headers":{"products":["product_id","product_name","description"]},"rows":{"products":[[1,"Widget A","This is a sample product with SN1234-5678"],[2,"Widget B","A product with serial SN9876-1234 in the description"],[3,"Widget C","Product SN1234-56789 is available now"],[4,"Widget D","No serial number here"],[5,"Widget E","Check out SN4321-8765 in this description"]]}}

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

Define state (indices, window, stack, map, DP cell, or recursion frame).
Apply one transition step and update the invariant.
Record answer candidate when condition is met.
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 #3465: Find Products with Valid Serial Numbers
// Auto-generated Java example from py.
class Solution {
    public void exampleSolution() {
    }
}
// Reference (py):
// # Accepted solution for LeetCode #3465: Find Products with Valid Serial Numbers
// import pandas as pd
// 
// 
// def find_valid_serial_products(products: pd.DataFrame) -> pd.DataFrame:
//     valid_pattern = r"\bSN[0-9]{4}-[0-9]{4}\b"
//     valid_products = products[
//         products["description"].str.contains(valid_pattern, regex=True)
//     ]
//     valid_products = valid_products.sort_values(by="product_id")
//     return valid_products

// Accepted solution for LeetCode #3465: Find Products with Valid Serial Numbers
// Auto-generated Go example from py.
func exampleSolution() {
}
// Reference (py):
// # Accepted solution for LeetCode #3465: Find Products with Valid Serial Numbers
// import pandas as pd
// 
// 
// def find_valid_serial_products(products: pd.DataFrame) -> pd.DataFrame:
//     valid_pattern = r"\bSN[0-9]{4}-[0-9]{4}\b"
//     valid_products = products[
//         products["description"].str.contains(valid_pattern, regex=True)
//     ]
//     valid_products = valid_products.sort_values(by="product_id")
//     return valid_products

# Accepted solution for LeetCode #3465: Find Products with Valid Serial Numbers
import pandas as pd


def find_valid_serial_products(products: pd.DataFrame) -> pd.DataFrame:
    valid_pattern = r"\bSN[0-9]{4}-[0-9]{4}\b"
    valid_products = products[
        products["description"].str.contains(valid_pattern, regex=True)
    ]
    valid_products = valid_products.sort_values(by="product_id")
    return valid_products

// Accepted solution for LeetCode #3465: Find Products with Valid Serial Numbers
// Rust example auto-generated from py reference.
// Replace the signature and local types with the exact LeetCode harness for this problem.
impl Solution {
    pub fn rust_example() {
        // Port the logic from the reference block below.
    }
}

// Reference (py):
// # Accepted solution for LeetCode #3465: Find Products with Valid Serial Numbers
// import pandas as pd
// 
// 
// def find_valid_serial_products(products: pd.DataFrame) -> pd.DataFrame:
//     valid_pattern = r"\bSN[0-9]{4}-[0-9]{4}\b"
//     valid_products = products[
//         products["description"].str.contains(valid_pattern, regex=True)
//     ]
//     valid_products = valid_products.sort_values(by="product_id")
//     return valid_products

// Accepted solution for LeetCode #3465: Find Products with Valid Serial Numbers
// Auto-generated TypeScript example from py.
function exampleSolution(): void {
}
// Reference (py):
// # Accepted solution for LeetCode #3465: Find Products with Valid Serial Numbers
// import pandas as pd
// 
// 
// def find_valid_serial_products(products: pd.DataFrame) -> pd.DataFrame:
//     valid_pattern = r"\bSN[0-9]{4}-[0-9]{4}\b"
//     valid_products = products[
//         products["description"].str.contains(valid_pattern, regex=True)
//     ]
//     valid_products = valid_products.sort_values(by="product_id")
//     return valid_products

Step 06

Interactive Study Demo

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

Custom checkpoints (one per line)

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.