2. Terms on leetcode

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⭐ String / Array rearrangement terms

Term	Meaning	Example
Permutation	Rearrangement of same elements	abc → bac
Combination	Choose elements, order doesn’t matter	{1,2} same as {2,1}
Anagram	String permutation (same chars, diff order)	listen → silent
Subsequence	Keep order, can skip chars	ace from abcde
Substring	Continuous part of string	bcd from abcde
Subarray	Continuous part of array	[2,3] from [1,2,3,4]
Subset	Any selected elements	{1,3} from {1,2,3}

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⭐ Super important distinctions (easy to confuse) 📌

Often confused	Difference
Substring vs Subsequence	continuous vs can skip
Subset vs Subarray	any elements vs continuous
Permutation vs Combination	order matters vs not

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⭐ Permutation vs Combination

1️⃣ Permutation = Arrange

When order matters.

From {A,B,C}, pick 2:

• AB
• BA
• AC
• CA
• BC
• CB

AB ≠ BA

So different order = different answer.

**Formula

P(n,r) = n!  /  (n-r)!

Example:

P(3,2) = 6

Used in:

• Arrange numbers
• Generate sequences
• Next permutation
• Anagrams / string rearrangement

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2️⃣ Combination = Choose

When order does NOT matter.

From {A,B,C}, choose 2:

• {A,B}
• {A,C}
• {B,C}

{A,B} = {B,A}

Same group.

Formula

C(n,r) = n! / r! (n-r)!

Example:

C(3,2) = 3

Used in:

• Select team members
• Pick subsets
• Combination sum
• Choose k elements

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3️⃣ Easy Memory Trick 📌

Permutation = Position / Arrange
→ order matters

Combination = Collect / Choose
→ order doesn’t matter

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Quick check

From {1,2,3}, pick 2:

Permutation

• 12
• 21
• 13
• 31
• 23
• 32

Combination

• {1,2}
• {1,3}
• {2,3}

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