Overview

Relevant problems for myself

• 🎨 Guides
• ⚡ Solutions
• ♿ Patterns

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