leetcode/docs/index.mdx

---
title: Introduction
description: Prad's Random Leetcode Notes
---

## Overview

This is a collection of random CS notes that I've made over the years. I've been learning CS for a while now and I thought it would be a good idea to make a public repository of my notes. This is the first iteration of the repository and I plan to add more content to it over time.

## The Golden Rule (Memorize This)

**Only sort if the problem asks for ORDER or RANKING.**

If the problem asks for:
- "first"
- "index"
- "position"
- "does it exist"
- "true/false"
- "count how many"

→ **DO NOT SORT**. Just use the frequency map and loop.

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## Decision Cheat Sheet

| What the problem wants                  | Do you need to sort? | What to do instead                  | Example Problem     |
|-----------------------------------------|----------------------|-------------------------------------|---------------------|
| Sort characters by how often they appear | YES                 | Freq + Sort                         | LC 451             |
| Top K most frequent                     | YES                 | Freq + Sort + slice                 | LC 347             |
| Sort array by frequency                 | YES                 | Freq + Sort                         | LC 1636            |
| First unique character (index)          | NO                  | Freq + loop original string         | LC 387             |
| Is it an anagram?                       | NO                  | Freq + check counts                 | LC 242             |
| How many numbers smaller than current   | Kind of             | Freq + prefix count                 | LC 1365            |
| Most frequent element                   | NO                  | Freq + find max                     | LC 169             |

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## Mental Checklist (Say this out loud every time)

1. "Did I count the frequencies?" → Yes
2. "Does the problem care about the **original order**?"  
   - Yes → Loop the original string/array and check freq
   - No  → You can sort
3. "Does it want an **index** or a **boolean**?"  
   - Yes → Almost never sort
4. "Does it want the elements **re-ordered**?"  
   - Yes → Sort by frequency

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## Super Short Version to Tattoo in Your Brain

**"If it wants the first / index / true-false → loop original.  
If it wants sorted / top k / reordered → sort."**

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## Practice Trigger Words

- "first non-repeating" → Loop original
- "return its index" → Loop original  
- "sort by frequency" → Sort
- "top k frequent" → Sort + slice
- "valid anagram" → Just check counts